Mark Pederson - Academia.edu (original) (raw)
Papers by Mark Pederson
Physical Review B, 1993
Results from quasidynamical local-density simulations on a small fullerene complex (Cps), three e... more Results from quasidynamical local-density simulations on a small fullerene complex (Cps), three endohedral complexes (C2sC, C2s@IZr, and CpsTi), a C atom bound to the corner of the C2s molecule, and Cp8H4 are presented. This molecule forms spontaneously from a 29-atom diamond crystallite and, under proper circumstances, is a covalent container compound. Upon encapsulation of a Zr atom, 12.6 eV of energy is liberated leading to an unreactive closed-shell C28Zr molecule. The open-shell structure of the empty molecule leads to a reactive yet relatively stable building block which might be useful for synthesis of new metastable forms of carbon-based materials. Electronic structures, ionization energies, electron afBnities, equilibrium geometries, and bare Hubbard U parameters are presented.
Current Opinion in Solid State and Materials Science, 2010
We employ the full-potential linearized augmented plane wave plus local orbital (FP-L/APW+lo) met... more We employ the full-potential linearized augmented plane wave plus local orbital (FP-L/APW+lo) method based on spin-polarized density functional theory (DFT) in order to investigate the structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Zn1−xCrxS and Cd1−xCrxS (x=0.25) in the zinc blende (B3) phase. For the exchange-correlation functional, the generalized gradient approximation GGA (Wu-Cohen 06) has been used. Results of calculated electronic band structures and density of states of these dilute magnetic semiconductors (DMSs) are discussed in terms of the contribution of Cr 3d5 4s1, Zn 3d10 4s2, Cd 4d10 5s2 and S 3s2 3p4 partial density of states. For Cr-based systems without n or p-type doping, the stability of the ferromagnetic spin state versus the antiferromagnetic state is predicted. Band structure and density of states studies show half-metallic ferromagnetic nature for both alloys, Zn1−xCrxS and Cd1−xCrxS. Calculations of the s–d exchange constant N0α and p–d exchange constant N0β clearly indicate the magnetic nature of these compounds. From the calculated total magnetic moments we observe that p–d hybridization reduces the local magnetic moment of Cr from its free space charge value and produces small local magnetic moments on the nonmagnetic Zn, Cd and S sites. The robustness of half-metallicity of Zn1−xCrxS and Cd1−xCrxS as a function of lattice constant is also discussed.
Applied Physics Letters, 2002
Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of all... more Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of alloying on the magnetic moment and magnetic anisotropies. Our density-functional studies show that many mixed clusters have moments comparable to or higher than the pure clusters. The mixed clusters, however, have very low anisotropies and could be ideal as soft magnetic materials. It is shown that shape, composition, and compositional ordering must be considered for optimization of anisotropy energies.
Journal of Computational Methods in Sciences and Engineering, 2008
We present a detailed investigation of static dipole polarizability of lithium clusters containin... more We present a detailed investigation of static dipole polarizability of lithium clusters containing up to 22 atoms. We first build a database of lithium clusters by optimizing several candidate structures for the ground state geometry for each size. The full polarizability tensor is determined for about 5-6 isomers of each cluster size using the finite-field method. All calculations are performed using large Gaussian basis sets, and within the generalized gradient approximation to the density functional theory, as implemented in the NRLMOL suite of codes. The average polarizability per atom varies from 11 to 9Å 3 within the 8-22 size range and show smoother decrease with increase in cluster size than the experimental values. While the average polarizability exhibits a relatively weak dependence on cluster conformation, significant changes in the degree of anisotropy of the polarizability tensor are observed. Interestingly, in addition to the expected even odd (0 and 1 µB) magnetic states, our results show several cases where clusters with an odd number of Li atoms exhibit elevated spin states (e.g. 3 µB).
Magnetochemistry, 2017
Without self-interaction corrections or the use of hybrid functionals, approximations to the dens... more Without self-interaction corrections or the use of hybrid functionals, approximations to the density-functional theory (DFT) often favor intermediate spin systems over high-spin systems. In this paper, we apply the recently proposed Fermi-Löwdin-orbital self-interaction corrected density functional formalism to a simple tetra-coordinated Fe(II)-porphyrin molecule and show that the energetic orderings of the S = 1 and S = 2 spin states are changed qualitatively relative to the results of Generalized Gradient Approximation (developed by Perdew, Burke, and Ernzerhof, PBE-GGA) and Local Density Approximation (developed by Perdew and Wang, PW92-LDA). Because the energetics, associated with changes in total spin, are small, we have also calculated the second-order spin-orbit energies and the zero-point vibrational energies to determine whether such corrections could be important in metal-substituted porphins. Our results find that the size of the spin-orbit and vibrational corrections to the energy orderings are small compared to the changes due to the self-interaction correction. Spin dependencies in the Infrared (IR)/Raman spectra and the zero-field splittings are provided as a possible means for identifying the spin in porphyrins containing Fe(II).
Physical Review B, 1999
We have used a recently developed computational technique based on density-functional theory to s... more We have used a recently developed computational technique based on density-functional theory to study the Raman-active modes of amorphous GeSe 2 and GeS 2. Vibrational modes and the associated Raman activities for three cluster building blocks of the glasses are calculated directly from first principles. The positions of the calculated symmetric-stretch modes in the cluster models are in excellent agreement with sharp features in the observed spectra. Moreover, simulated spectra based on the cluster results are in good agreement with experiment, accounting for all the observed features in the bond-stretch region of the spectra. The cluster results suggest a new interpretation for the 250 cm Ϫ1 mode appearing in the spectra of Ge-rich samples in the Ge x S 1Ϫx family. ͓S0163-1829͑99͒51846-8͔
Physical Review B, 1997
We have used a density-functional-based approach to study the response of silicon clusters to app... more We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared ͑IR͒ intensities for all of the vibrational modes of several clusters ͑Si N with Nϭ3Ϫ8, 10, 13, 20, and 21͒ using the local density approximation ͑LDA͒. For the smaller clusters (Nϭ3Ϫ8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si 10 and for the lowest-energy structure of Si 13 found to date. For the static response, we compare our calculated polarizabilities for Nϭ10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems. ͓S0163-1829͑97͒01303-9͔
The Journal of Physical Chemistry, 1994
We present a simple model for understanding the bonding of endohedral atoms in small fullerene ca... more We present a simple model for understanding the bonding of endohedral atoms in small fullerene cages, based on their approximate spherical shape. Previous work has shown that the one-electron wave functions of a fullerene cage can be assigned angular momentum quantum numbers which describe their overall angular character. These quantum numbers form the basis for approximate selection rules which govern the bonding with endohedral atoms. With this model we successfully address thevery different bonding ofvarious tetravalent elements in Cza and the remarkably strong bonding of U in this small fullerene. We also make several specific predictions regarding the stability of other endohedral complexes.
![Research paper thumbnail of Erratum: Simplified generalized-gradient approximation and anharmonicity: Benchmark calculations on molecules [Phys. Rev. B 55 , 7454 (1997)]](https://attachments.academia-assets.com/116360106/thumbnails/1.jpg)
](Physical Review B, 1998
In regard to Table IV in the above paper, several errors were present. The Raman activity for the... more In regard to Table IV in the above paper, several errors were present. The Raman activity for the H 2 molecule should be 161.4725. Also, a nonconventional manner of presenting the intensities of degenerate modes for CH 4 was used. Both the T 2 modes should be multiplied by 3 and the E 1 mode should be doubled to put them in a more conventional form. In addition, a subtle numerical error led to incorrect values for the CO 2 molecule. The values should appear as below: CO 2
Physical Review B, 2002
We report the electronic structure and magnetic ordering of the single molecule magnet [Mn10O4(2,... more We report the electronic structure and magnetic ordering of the single molecule magnet [Mn10O4(2,2'biphenoxide)4Br12] 4− based on first-principles all-electron density-functional calculations. We find that two of the ten core Mn atoms are coupled antiferromagnetically to the remaining eight, resulting in a ferrimagnetic ground state with total spin S = 13. The calculated magnetic anisotropy barrier is found to be 9 K in good agreement with experiment. The presence of the Br anions impact the electronic structure and therefore the magnetic properties of the 10 Mn atoms. However, the electric field due to the negative charges has no significant effect on the magnetic anisotropy.
Physical Review B, 1988
An F center in the lithium fluoride (LiF) crystal is investigated with use of the muSn-tin Green ... more An F center in the lithium fluoride (LiF) crystal is investigated with use of the muSn-tin Green s-function formalism and a linear combination of atomic orbitals (LCAO) cluster method. Both of these methods properly embed the defect into the host crystal. With the latter method, we are able to include the defect-induced charge relaxation on many shells of nearest-neighbor atoms. We have employed several variants of the density-functional approximation which allow a more accurate description of the ground-state single-electron properties. These methods include an empirical adjustment of the perfect-crystal band gap [the "LDA scissor operator" (where LDA denotes local-density approximation)] as well as the inclusion of electronic self-interaction corrections. Since the validity of the density-functional formalism is questionable for obtaining excited-state properties, we have introduced a single particle-hole excited-state theory, which is based on manyelectron arguments, in order to gain insight about the behavior of the excited-state effective potentials. We demonstrate that, in contrast to LDA, for localized excitations the long-range behavior of the effective excited-state potential should exhibit a-1/r tail in neutral systems. While the qualitative behavior of the LDA potential differs from the effective excited-state potential, the selfinteraction-corrected LDA potential of the highest occupied defect level exhibits the correct qualitative behavior. By allowing the excited state electron to move in the latter potential, an accurate excitation energy for the a&~-t &"absorption in LiF is obtained. Further, in contrast to the densityfunctional results, the self-interaction-corrected version of the theory correctly places the t, "state below the conduction-band edge.
Physical Review B, 1990
We make use of the existing formalism for calculating atomic forces within the local-density appr... more We make use of the existing formalism for calculating atomic forces within the local-density approximation {LDA) to determine forces in an all-electron, local-orbital framework. The forces are calculated as the proper total-energy derivatives, including the necessary basis-set corrections. In our computational technique the LDA potential is evaluated exactly on a variationally determined integration mesh which allows all integrals relevant to the electronic-structure problem to be computed to any desired accuracy. We apply our technique to the molybdenum dimer, the ethylene molecule, and the NaCl monomer to demonstrate the high accuracy of the calculated forces. In addition, we investigate directly the effects of integration and self-consistency errors on the accuracy of the forces, and use the results of these tests to suggest strategies for the eScient use of force information in structural studies.
Physical Review Letters, 1991
The binding energy of the diamond 1s core exciton has a measured value of 0.2 eV, in marked contr... more The binding energy of the diamond 1s core exciton has a measured value of 0.2 eV, in marked contrast to the 1.7-eV binding energy of the physically similar nitrogen donor in diamond. Using the localdensity approximation to study the core exciton, we find the shallow exciton binding energy is explained by dipole selection rules which prohibit occupation of a deep exciton gap state in single-photon excitation. This deep state has not yet been observed experimentally, and the calculation predicts a binding energy in the range of 0.8-1.7 eV.
Physical Review B, 1994
Boron hydride analogues of the fullerenes
Physical Review B, 1994
Total-energy calculations for 13 difFerent fullerene clusters, including 8 toroidal fullerenes, a... more Total-energy calculations for 13 difFerent fullerene clusters, including 8 toroidal fullerenes, are performed using a parametrized tight-binding functional, and in select cases, an all-electron firstprinciples local-density functional. The tori considered here are composed of graphitelike sixmembered rings, with five-membered rings on the outer surface of the torus providing positive Gaussian curvature, and seven-membered rings on the inner toroidal surface providing negative Gaussian curvature. Comparisons are made with energies calculated from a Stillinger-Webertype potential for graphite, which is found to be inadequate for addressing relative energetic stability among the tori. Two empirical strain relations are tested against our tight-binding energy calculations. A simple elastic theory is found to give reasonably good energy orderings. Charge-density calculations on the tori reveal that electron density is enhanced for atoms in five-membered rings, and depleted for atoms in seven-membered rings. First-principles total-energy calculations indicate that a single potassium atom is bound to the center of a C&44 torus by 0.5 eV. It is proposed that atoms with higher orbital angular momentum (e.g., Zr, etc.) may bond more tightly to the torus, and thus facilitate formation of Cqqo or similar tori.
Physical Review B, 1992
We have used our local-orbital cluster codes to perform detailed density-functional-based calcula... more We have used our local-orbital cluster codes to perform detailed density-functional-based calculations on isolated C60 molecules. We present firstand second-electron affinities, which include all effects due to spin polarization and charge-induced geometrical relaxation. The effects due to the generalized gradient approximation are reported as well. The two Ag and single A"vibrational modes are presented and frequency shifts due to charging are estimated. By placing the fullerene molecules in a static electric field of variable strength, the molecular static polarizabilities are obtained. In comparison to isolated carbon atoms, we find enhancements in the linear polarizabilities due to the delocalized electrons at the Fermi level, but do not observe any large nonlinear static contributions. By including effects due to charge transfer, on-site geometric relaxation, and fullerene polarization, we introduce a simple potential that accounts for long-range interactions and predict Hubbard parameters as a function of alkali-dopant concentration.
Physical Review Letters, 1992
Fullerene tubules are shown to be highly polarizable "molecular straws" capable of ingesting dipo... more Fullerene tubules are shown to be highly polarizable "molecular straws" capable of ingesting dipolar molecules. Local-density-functional calculations on HF molecules within a finite-length tubule, of size 144 atoms, demonstrate this eAect. The energy of incarceration is several times the thermal ambient at room temperature. These calculations, now feasible on desktop workstations, open the way to the study of nanoscale capillarity and to, perhaps, precise control over shielding of specific "guest" compounds from external electric and magnetic fields.
Physical Review Letters, 1985
This document has been approved fir public release and sale; its Idi~tyjbutiorx is unlimited.
Physical Review B, 1993
Results from quasidynamical local-density simulations on a small fullerene complex (Cps), three e... more Results from quasidynamical local-density simulations on a small fullerene complex (Cps), three endohedral complexes (C2sC, C2s@IZr, and CpsTi), a C atom bound to the corner of the C2s molecule, and Cp8H4 are presented. This molecule forms spontaneously from a 29-atom diamond crystallite and, under proper circumstances, is a covalent container compound. Upon encapsulation of a Zr atom, 12.6 eV of energy is liberated leading to an unreactive closed-shell C28Zr molecule. The open-shell structure of the empty molecule leads to a reactive yet relatively stable building block which might be useful for synthesis of new metastable forms of carbon-based materials. Electronic structures, ionization energies, electron afBnities, equilibrium geometries, and bare Hubbard U parameters are presented.
Current Opinion in Solid State and Materials Science, 2010
We employ the full-potential linearized augmented plane wave plus local orbital (FP-L/APW+lo) met... more We employ the full-potential linearized augmented plane wave plus local orbital (FP-L/APW+lo) method based on spin-polarized density functional theory (DFT) in order to investigate the structural, electronic and magnetic properties of ordered dilute ferromagnetic semiconductors Zn1−xCrxS and Cd1−xCrxS (x=0.25) in the zinc blende (B3) phase. For the exchange-correlation functional, the generalized gradient approximation GGA (Wu-Cohen 06) has been used. Results of calculated electronic band structures and density of states of these dilute magnetic semiconductors (DMSs) are discussed in terms of the contribution of Cr 3d5 4s1, Zn 3d10 4s2, Cd 4d10 5s2 and S 3s2 3p4 partial density of states. For Cr-based systems without n or p-type doping, the stability of the ferromagnetic spin state versus the antiferromagnetic state is predicted. Band structure and density of states studies show half-metallic ferromagnetic nature for both alloys, Zn1−xCrxS and Cd1−xCrxS. Calculations of the s–d exchange constant N0α and p–d exchange constant N0β clearly indicate the magnetic nature of these compounds. From the calculated total magnetic moments we observe that p–d hybridization reduces the local magnetic moment of Cr from its free space charge value and produces small local magnetic moments on the nonmagnetic Zn, Cd and S sites. The robustness of half-metallicity of Zn1−xCrxS and Cd1−xCrxS as a function of lattice constant is also discussed.
Applied Physics Letters, 2002
Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of all... more Electronic structure calculations of FenCom (n+m=5 and 13) are used to examine the effects of alloying on the magnetic moment and magnetic anisotropies. Our density-functional studies show that many mixed clusters have moments comparable to or higher than the pure clusters. The mixed clusters, however, have very low anisotropies and could be ideal as soft magnetic materials. It is shown that shape, composition, and compositional ordering must be considered for optimization of anisotropy energies.
Journal of Computational Methods in Sciences and Engineering, 2008
We present a detailed investigation of static dipole polarizability of lithium clusters containin... more We present a detailed investigation of static dipole polarizability of lithium clusters containing up to 22 atoms. We first build a database of lithium clusters by optimizing several candidate structures for the ground state geometry for each size. The full polarizability tensor is determined for about 5-6 isomers of each cluster size using the finite-field method. All calculations are performed using large Gaussian basis sets, and within the generalized gradient approximation to the density functional theory, as implemented in the NRLMOL suite of codes. The average polarizability per atom varies from 11 to 9Å 3 within the 8-22 size range and show smoother decrease with increase in cluster size than the experimental values. While the average polarizability exhibits a relatively weak dependence on cluster conformation, significant changes in the degree of anisotropy of the polarizability tensor are observed. Interestingly, in addition to the expected even odd (0 and 1 µB) magnetic states, our results show several cases where clusters with an odd number of Li atoms exhibit elevated spin states (e.g. 3 µB).
Magnetochemistry, 2017
Without self-interaction corrections or the use of hybrid functionals, approximations to the dens... more Without self-interaction corrections or the use of hybrid functionals, approximations to the density-functional theory (DFT) often favor intermediate spin systems over high-spin systems. In this paper, we apply the recently proposed Fermi-Löwdin-orbital self-interaction corrected density functional formalism to a simple tetra-coordinated Fe(II)-porphyrin molecule and show that the energetic orderings of the S = 1 and S = 2 spin states are changed qualitatively relative to the results of Generalized Gradient Approximation (developed by Perdew, Burke, and Ernzerhof, PBE-GGA) and Local Density Approximation (developed by Perdew and Wang, PW92-LDA). Because the energetics, associated with changes in total spin, are small, we have also calculated the second-order spin-orbit energies and the zero-point vibrational energies to determine whether such corrections could be important in metal-substituted porphins. Our results find that the size of the spin-orbit and vibrational corrections to the energy orderings are small compared to the changes due to the self-interaction correction. Spin dependencies in the Infrared (IR)/Raman spectra and the zero-field splittings are provided as a possible means for identifying the spin in porphyrins containing Fe(II).
Physical Review B, 1999
We have used a recently developed computational technique based on density-functional theory to s... more We have used a recently developed computational technique based on density-functional theory to study the Raman-active modes of amorphous GeSe 2 and GeS 2. Vibrational modes and the associated Raman activities for three cluster building blocks of the glasses are calculated directly from first principles. The positions of the calculated symmetric-stretch modes in the cluster models are in excellent agreement with sharp features in the observed spectra. Moreover, simulated spectra based on the cluster results are in good agreement with experiment, accounting for all the observed features in the bond-stretch region of the spectra. The cluster results suggest a new interpretation for the 250 cm Ϫ1 mode appearing in the spectra of Ge-rich samples in the Ge x S 1Ϫx family. ͓S0163-1829͑99͒51846-8͔
Physical Review B, 1997
We have used a density-functional-based approach to study the response of silicon clusters to app... more We have used a density-functional-based approach to study the response of silicon clusters to applied electric fields. For the dynamical response, we have calculated the Raman activities and infrared ͑IR͒ intensities for all of the vibrational modes of several clusters ͑Si N with Nϭ3Ϫ8, 10, 13, 20, and 21͒ using the local density approximation ͑LDA͒. For the smaller clusters (Nϭ3Ϫ8) our results are in good agreement with previous quantum-chemical calculations and experimental measurements, establishing that LDA-based IR and Raman data can be used in conjunction with measured spectra to determine the structure of clusters observed in experiment. To illustrate the potential of the method for larger clusters, we present calculated IR and Raman data for two low-energy isomers of Si 10 and for the lowest-energy structure of Si 13 found to date. For the static response, we compare our calculated polarizabilities for Nϭ10, 13, 20, and 21 to recent experimental measurements. The calculated results are in rough agreement with experiment, but show less variation with cluster size than the measurements. Taken together, our results show that LDA calculations can offer a powerful means for establishing the structures of experimentally fabricated clusters and nanoscale systems. ͓S0163-1829͑97͒01303-9͔
The Journal of Physical Chemistry, 1994
We present a simple model for understanding the bonding of endohedral atoms in small fullerene ca... more We present a simple model for understanding the bonding of endohedral atoms in small fullerene cages, based on their approximate spherical shape. Previous work has shown that the one-electron wave functions of a fullerene cage can be assigned angular momentum quantum numbers which describe their overall angular character. These quantum numbers form the basis for approximate selection rules which govern the bonding with endohedral atoms. With this model we successfully address thevery different bonding ofvarious tetravalent elements in Cza and the remarkably strong bonding of U in this small fullerene. We also make several specific predictions regarding the stability of other endohedral complexes.
Physical Review B, 1998
In regard to Table IV in the above paper, several errors were present. The Raman activity for the... more In regard to Table IV in the above paper, several errors were present. The Raman activity for the H 2 molecule should be 161.4725. Also, a nonconventional manner of presenting the intensities of degenerate modes for CH 4 was used. Both the T 2 modes should be multiplied by 3 and the E 1 mode should be doubled to put them in a more conventional form. In addition, a subtle numerical error led to incorrect values for the CO 2 molecule. The values should appear as below: CO 2
Physical Review B, 2002
We report the electronic structure and magnetic ordering of the single molecule magnet [Mn10O4(2,... more We report the electronic structure and magnetic ordering of the single molecule magnet [Mn10O4(2,2'biphenoxide)4Br12] 4− based on first-principles all-electron density-functional calculations. We find that two of the ten core Mn atoms are coupled antiferromagnetically to the remaining eight, resulting in a ferrimagnetic ground state with total spin S = 13. The calculated magnetic anisotropy barrier is found to be 9 K in good agreement with experiment. The presence of the Br anions impact the electronic structure and therefore the magnetic properties of the 10 Mn atoms. However, the electric field due to the negative charges has no significant effect on the magnetic anisotropy.
Physical Review B, 1988
An F center in the lithium fluoride (LiF) crystal is investigated with use of the muSn-tin Green ... more An F center in the lithium fluoride (LiF) crystal is investigated with use of the muSn-tin Green s-function formalism and a linear combination of atomic orbitals (LCAO) cluster method. Both of these methods properly embed the defect into the host crystal. With the latter method, we are able to include the defect-induced charge relaxation on many shells of nearest-neighbor atoms. We have employed several variants of the density-functional approximation which allow a more accurate description of the ground-state single-electron properties. These methods include an empirical adjustment of the perfect-crystal band gap [the "LDA scissor operator" (where LDA denotes local-density approximation)] as well as the inclusion of electronic self-interaction corrections. Since the validity of the density-functional formalism is questionable for obtaining excited-state properties, we have introduced a single particle-hole excited-state theory, which is based on manyelectron arguments, in order to gain insight about the behavior of the excited-state effective potentials. We demonstrate that, in contrast to LDA, for localized excitations the long-range behavior of the effective excited-state potential should exhibit a-1/r tail in neutral systems. While the qualitative behavior of the LDA potential differs from the effective excited-state potential, the selfinteraction-corrected LDA potential of the highest occupied defect level exhibits the correct qualitative behavior. By allowing the excited state electron to move in the latter potential, an accurate excitation energy for the a&~-t &"absorption in LiF is obtained. Further, in contrast to the densityfunctional results, the self-interaction-corrected version of the theory correctly places the t, "state below the conduction-band edge.
Physical Review B, 1990
We make use of the existing formalism for calculating atomic forces within the local-density appr... more We make use of the existing formalism for calculating atomic forces within the local-density approximation {LDA) to determine forces in an all-electron, local-orbital framework. The forces are calculated as the proper total-energy derivatives, including the necessary basis-set corrections. In our computational technique the LDA potential is evaluated exactly on a variationally determined integration mesh which allows all integrals relevant to the electronic-structure problem to be computed to any desired accuracy. We apply our technique to the molybdenum dimer, the ethylene molecule, and the NaCl monomer to demonstrate the high accuracy of the calculated forces. In addition, we investigate directly the effects of integration and self-consistency errors on the accuracy of the forces, and use the results of these tests to suggest strategies for the eScient use of force information in structural studies.
Physical Review Letters, 1991
The binding energy of the diamond 1s core exciton has a measured value of 0.2 eV, in marked contr... more The binding energy of the diamond 1s core exciton has a measured value of 0.2 eV, in marked contrast to the 1.7-eV binding energy of the physically similar nitrogen donor in diamond. Using the localdensity approximation to study the core exciton, we find the shallow exciton binding energy is explained by dipole selection rules which prohibit occupation of a deep exciton gap state in single-photon excitation. This deep state has not yet been observed experimentally, and the calculation predicts a binding energy in the range of 0.8-1.7 eV.
Physical Review B, 1994
Boron hydride analogues of the fullerenes
Physical Review B, 1994
Total-energy calculations for 13 difFerent fullerene clusters, including 8 toroidal fullerenes, a... more Total-energy calculations for 13 difFerent fullerene clusters, including 8 toroidal fullerenes, are performed using a parametrized tight-binding functional, and in select cases, an all-electron firstprinciples local-density functional. The tori considered here are composed of graphitelike sixmembered rings, with five-membered rings on the outer surface of the torus providing positive Gaussian curvature, and seven-membered rings on the inner toroidal surface providing negative Gaussian curvature. Comparisons are made with energies calculated from a Stillinger-Webertype potential for graphite, which is found to be inadequate for addressing relative energetic stability among the tori. Two empirical strain relations are tested against our tight-binding energy calculations. A simple elastic theory is found to give reasonably good energy orderings. Charge-density calculations on the tori reveal that electron density is enhanced for atoms in five-membered rings, and depleted for atoms in seven-membered rings. First-principles total-energy calculations indicate that a single potassium atom is bound to the center of a C&44 torus by 0.5 eV. It is proposed that atoms with higher orbital angular momentum (e.g., Zr, etc.) may bond more tightly to the torus, and thus facilitate formation of Cqqo or similar tori.
Physical Review B, 1992
We have used our local-orbital cluster codes to perform detailed density-functional-based calcula... more We have used our local-orbital cluster codes to perform detailed density-functional-based calculations on isolated C60 molecules. We present firstand second-electron affinities, which include all effects due to spin polarization and charge-induced geometrical relaxation. The effects due to the generalized gradient approximation are reported as well. The two Ag and single A"vibrational modes are presented and frequency shifts due to charging are estimated. By placing the fullerene molecules in a static electric field of variable strength, the molecular static polarizabilities are obtained. In comparison to isolated carbon atoms, we find enhancements in the linear polarizabilities due to the delocalized electrons at the Fermi level, but do not observe any large nonlinear static contributions. By including effects due to charge transfer, on-site geometric relaxation, and fullerene polarization, we introduce a simple potential that accounts for long-range interactions and predict Hubbard parameters as a function of alkali-dopant concentration.
Physical Review Letters, 1992
Fullerene tubules are shown to be highly polarizable "molecular straws" capable of ingesting dipo... more Fullerene tubules are shown to be highly polarizable "molecular straws" capable of ingesting dipolar molecules. Local-density-functional calculations on HF molecules within a finite-length tubule, of size 144 atoms, demonstrate this eAect. The energy of incarceration is several times the thermal ambient at room temperature. These calculations, now feasible on desktop workstations, open the way to the study of nanoscale capillarity and to, perhaps, precise control over shielding of specific "guest" compounds from external electric and magnetic fields.
Physical Review Letters, 1985
This document has been approved fir public release and sale; its Idi~tyjbutiorx is unlimited.