Theory and energetics of mass spectra (original) (raw)
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Physical Review A, 1992
The Kohn-Sham energy with exact exchange [using the exact Hartree-Fock (HF) exchange but an approximate correlation-energy functional] may be computed very accurately by adding the correlation obtained from the HF density to the total HF energy. Three density functionals are used: local spin density (LSD), LSD with self-interaction correction, and LSD with generalized gradient correction. This scheme has been extended (Lie-Clementi, Colle-Salvetti, and Moscardo-San-Fabian) to be used with general-valence-bond (GVB) energies and wave functions, so that the extra correlation included in the GVB energy is not counted again. The effect of all these approximate correlations on HF or GVB spectroscopic constants (R"co"and D,) is studied. Approximate relations showing how correlation affects them are derived, and may be summarized as follows: (1) the effect on R, and co, depends only on the correlation derivative at R"and (2) the effect on D, depends mainly on the correlation difference between quasidissociated and equilibrium geometries. A consequence is that all correlation corrections tested here give larger co, and D, and shorter R, than the uncorrected HF or GVB values. This trend is correct for D, for both HF and GVB. For R, and co" it is correct in most cases for GVB, but it often fails for the HF cases. A comparison is made with Kohn-Sham calculations with both exchange and correlation approximated. As a final conclusion, it is found that, within the present scheme, a qualitatively correct HF or GVB potential-energy curve, together with a correlation-energy approximation with correct dissociation behavior, is crucial for obtaining good estimates of spectroscopic constants.
Journal of Electron Spectroscopy and Related Phenomena, 2009
In this paper, ionization energies of gas-phase atoms and molecules are calculated by energy-difference method and by approximate transition-state models with density functional theory (DFT). To determine the best functionals for ionization energies, we first study the H to Ar atoms. An approximation is used in which the electron density is first obtained from Kohn-Sham computations with an exchangecorrelation potential V xc known as statistical average of orbital potentials (SAOP), after which the energy is computed from that density with 59 different exchange-correlation energy functionals E xc . For the 18 atoms, the best E xc functional providing an average absolute deviation (AAD) of only 0.110 eV is one known as the Krieger-Chen-Iafrate-Savin functional modified by Krieger, Chen, Iafrate, and Kurth, if one uses the spin-polarized spherical atom description. On the other hand, if one imposes the condition of integer-electrons, the best functional is the Becke 1997 functional modified by Wilson, Bradley, and Tozer, with an AAD of 0.107 eV, while several other functionals perform almost as well. For molecules, we can achieve an accuracy of AAD = 0.21 eV for valence VIPs of nonperhalo molecules with E(V xc = SAOP;PBE0) using integer-electron description. For perhalo molecules our best approach is E(V xc from either E xc or SAOP;mPW1PW) with full symmetry to obtain an AAD = 0.24 eV.
Ab initio study of rovibronic energies of the CH 2 + molecular ion
Optics and Spectroscopy, 2008
The potential energy surfaces of the lowest electronic states 2 A 1 and 2 B 1 of the molecular ion are calculated in the second order of the perturbation theory with the reference function obtained by the multiconfiguration self-consistent-field method in the complete-active-space approximation. Based on the ab initio calculated potential energy surfaces, the rovibronic energies of are calculated by the variational method using the RENNER Hamiltonian. It is found that the accuracy of the perturbation theory method involving many reference configurations is as good as the accuracy of the best ab initio calculations performed by the configuration interaction method with many reference configurations and by the coupled cluster method with a single reference configuration. Empirical refinement of the two parameters of the potential function leads to good agreement between calculation and experiment, except for the wavenumbers of the (0, 3, 0) 1 (0, 0, 0) 0 transitions.
Published as part of The Journal of Physical Chemistry virtual special issue
Vertical and adiabatic excitation energies and oscillator strengths for valence and Rydberg states of hydroxycarbene (HCOH) and methylhydroxycarbene (CH 3 COH) are reported. The electronic properties were computed with equation-of-motion coupled-cluster methods with single and double substitution methods (EOM-CCSD) and the aug-cc-pVTZ basis set. The states' characters were analyzed by plotting natural transition orbitals (NTOs). The calculations demonstrate that the shape, size, and energy of each Rydberg orbital are affected to varying degrees by their interaction with the ion core. Likewise, the corresponding quantum defects reflect the Rydberg electron−ion core interactions. The results reported herein, combined with previously reported calculations of the photoelectron spectrum of HCOH, should help in designing strategies for state-selective detection of hydroxycarbenes via ionization.
International Journal of Quantum Chemistry, 2005
Contracted basis sets of double zeta valence quality plus polarization functions (DZP) and augmented DZP basis sets, which were recently constructed for the first-and second-row atoms, are applied to study the electronic ground states of the diatomic molecules CN Ϫ , N 2 , AlF, SiO, PN, SC, ClB, and P 2. At the Hartree-Fock (HF) and/or Møller-Plesset second-order (MP2) levels, total and molecular orbital energies, dissociation energies, bond lengths, harmonic vibrational frequencies, and dipole moments are calculated and compared with available experimental data and with the results obtained from correlation consistent polarized valence basis sets of Dunning's group. For N 2 , calculations of polarizabilities at the HF and MP2 levels with the sets presented above are also done and compared with results reported in the literature.
2004
Purpose With the trend towards needing information about chemistry at conditions significantly different from 298K and 1 atm., methods need to be developed to generate and interpret this data. This demand for information about chemistry at extreme conditions comes from many fields. The study of atmospheric chemistry requires knowledge of unusual species that are formed when molecules are exposed to ultraviolet radiation. Studying of energetic materials requires knowledge of the thermochemical and structural properties of a myriad of chemical species under a wide range of temperatures. Basic scientific understanding of the very nature of a chemical bond requires detailed information. Studying these problems computationally requires multiple capabilities. The methodology used must provide both high accuracy and computational efficiency. Studying extreme chemistry also suffers from all the challenges of studying chemistry under non-extreme conditions. Therefore, either a new method must be developed or an old method must be applied in an innovative way. The approach The method we have chosen to use is path integral Monte Carlo (PIMC) for the nuclear degrees of freedom and ab initio electronic structure methods for the electronic degrees of freedom. PIMC and ab initio electronic structure are methods of treating the quantum nature of particles. These methods have been chosen, because an accurate treatment requires treating both the electrons and the nuclei as quantum particles. We developed new "projected" methods that reduce the computational demands. These methods along with PIMC in general are described in two Journal of Chemical Physics articles (UCRL-JC-144960 and UCRL-JC-147423). This methodology was implemented into a PIMC code developed as part of this LDRD. The code was parallelized in order to utilize the computational resources of LLNL. Technical accomplishments By coupling quantum treatment of the electrons via ab initio method to the quantum motion of the nuclei with PIMC, a method that is able to produce accurate results is obtained. This level of accuracy is necessary under extreme conditions, because traditional "empirical" corrections are not applicable under such conditions. Because the electronic structure is tightly coupled to the motion of the nuclei, ab initio methods must be used to treat the electrons. These methods have explicit molecular orbital dependence are not parameterized to simply reproduce an equilibrium geometry. This method can calculate structural information, energetics, and heat capacities. The thermochemical results are described in detail in a Journal of Chemical Physics article (UCRL-JC-149046). Conclusion A theoretical method must provide the level of accuracy needed to answer the question of interest; otherwise, the results could be potentially misleading. This methodology used in this LDRD provided both structural and energetic information. The method used provides a fully quantum approach to calculation properties of molecules under extreme conditions. The software to perform these calculations is written and is integrated with a variety of electronic structure codes (GAMESS and Fireball). The code incorporates new computer time saving methodologies. This method calculates the properties needed, such as energy and heat capacity. The code continues to be used after the completion of the LDRD.
Journal of Molecular Modeling, 2014
In this work, we performed a thorough investigation of potential energy curves, rovibrational spectra, and spectroscopic constants for dimers whose interactions are mediated by hydrogen bonds and other hydrogen interactions. Particularly, we deal with CH 4 ⋯CH 4 , CH 4 ⋯H 2 O, CH 4 ⋯CHF 3 , and H 2 O⋯CHF 3 dimers by employing accurate electronic energy calculations with two different basis sets at the MP2 level of theory. Following this, the discrete variable representation method was applied to solve the nuclear Schrödinger equation, thus obtaining spectroscopic constants and rovibrational spectra. The harmonic constant, ω e , presents a direct relation to the strength of dimer interactions. As a general rule, it was found that a decrease of interatomic distances is followed by the increase of D e for all dimers. This behavior suggests that the interaction of CH 4 ⋯CH 4 is the weakest among all dimers, followed by CH 4 ⋯CHF 3 , CH 4 ⋯H 2 O and the strongest interaction given by the H 2 O⋯CHF 3 dimer.
J Phys Chem a, 1997
Using systematic sequences of the newly developed correlation consistent core-valence basis sets from cc-pCVDZ through cc-pCV6Z, the spectroscopic constants of the homonuclear diatomic molecules containing ®rst row atoms, B±F, are calculated both with and without inclusion of 1s correlation. Internally contracted multireference con®guration interaction (IC-MRCI) and singles and doubles coupled cluster (CCSD) theory with a perturbational estimate of connected triple excitations, CCSD(T), have been investigated. By exploiting the convergence of the correlation consistent basis sets, complete basis set (CBS) limits have been estimated for total energies, dissociation energies, equilibrium geometries, and harmonic frequencies.