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Papers by Omololu Akin-Ojo
Chemical Physics, 2011
The second virial coefficient, B12(T), of the H2O-H2 system has been calculated ab initio over th... more The second virial coefficient, B12(T), of the H2O-H2 system has been calculated ab initio over the temperature range 200-700 K. A semi-classical method was employed with two recent accurate potential energy surfaces. The agreement with experimental data is good, although experimental error bars are much larger than the theoretical uncertainties. We show that highly correlated potentials are required for an
This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using ... more This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using cyanoacetylene in helium and methane in water as our two main case studies. For the former system, information on the spectra was obtained, while the latter was used as a prototype to study the hydrophobic effect. The most important elements needed in the study of these systems, namely, the intermolecular potential energy surfaces (PESs) were developed from first principles, i.e., without the use of any experimental data. In the case of the CH4-H2O system, the PESs were also used to calculate the cross second virial coefficient, useful in the natural-gas industry. The predictions of the spectra of the He-HCCCN dimer made in this work have now been confirmed by experiments. By extending the results for this dimer using a simple model, the reduction in the rotational constant of HCCCN upon solvation in a superfluid helium nanodroplet was explained and the value of this constant was predicted. The work on the hydrophobic effect involved molecular dynamics simulations of methane-water mixtures. In some of these calculations, a polarization model was used to go beyond the popularly-used pairwise additivity approximation of the interaction energies for many-body systems. The results from the molecular simulations increase our understanding of the nature of the hydrophobic effect by describing the influence of temperature on this phenomenon and also shedding light on the relation between the hydrophobic effect and the molecular properties of the solute molecules. The need to use the Helmholtz free energy and not simply one of its components (the entropic one as is often done) to explain the aggregation of apolar molecules in water was also emphasized.
Chemical Physics, 2010
The force matching method is used to improve density functional theory (DFT) by designing a suppl... more The force matching method is used to improve density functional theory (DFT) by designing a supplemental potential to capture the difference in atomic forces between a DFT functional and a high-quality post Hartree-Fock method. The supplemental potential has two-body terms designed to correct for dispersion and hydrogen bond interactions. The potential also has one-body terms to improve the description of the intramolecular potential energy surface. Our procedure is tested by providing corrections to the Becke-Lee-Yang-Parr exchange-correlation functional for water and is found to perform significantly better than the standard DFT-D approach, giving QCISD quality predictions for relative cluster energies, atomic forces, and molecular structures. It is found that a simple Lennard-Jones term does a good job at correcting for van der Waals interactions and possibly also providing corrections to exchange repulsion. The one-body corrections, while contributing only slightly to improving relative cluster energies, significantly reduce the errors in binding energies and atomic forces for the systems studied.
Molecular Simulation, 2011
This article summarises the adaptive force matching (AFM) method recently developed by the Wang g... more This article summarises the adaptive force matching (AFM) method recently developed by the Wang group. The AFM method is capable of parameterising force fields in the condensed phase by fitting electronic structure forces obtained through quantum mechanics/molecular mechanics (QM/MM) calculations. The AFM method utilises an iterative procedure to ensure good sampling and high-quality MM models for QM/MM calculations. By fitting
Chemical Physics, 2008
A new method called adaptive force matching (AFM) has been developed that is capable of producing... more A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.
Journal of Computational Chemistry, 2011
The recently introduced adaptive force matching (AFM) method is used to develop a significantly i... more The recently introduced adaptive force matching (AFM) method is used to develop a significantly improved pair-wise nonpolarizable potential for water. A rigid version of the potential is also presented to enable larger time steps for biological simulations. In this work, it is demonstrated that the AFM method can be used to systematically assess the importance of each functional term during the construction of a force field. For a water potential, it is established that a single off-atom charge center (M) in the plane of water outperforms two out-of-plane charge sites for reproducing intermolecular forces. The four-site pair-wise nonpolarizable force field developed in this work rivals some of the most sophisticated polarizable models in terms of reproducing accurate ab initio forces. The force fields are parameterized to perform best in the temperature range from 0 to 40°C. Equilibrium and dynamical properties calculated with the flexible and rigid force fields are in good agreement with experimental results. For the flexible model, the agreement improves when path integral simulation is performed. These force fields provide high-quality results at a very low computational cost and are thus well suited to atomistic scale biological simulations. The AFM method provides a mechanism for selecting important terms in force field expressions and is a very promising tool for producing accurate force fields in condensed phases.
Journal of Physical Chemistry B, 2009
The MP2 f water force field, recently developed using the adaptive force matching method, is impr... more The MP2 f water force field, recently developed using the adaptive force matching method, is improved significantly after the introduction of a short-range repulsion term. This term is designed to address the deficiency of point-charge models at short charge-charge separations. It may also provide a more realistic description of hydrogen bonds by capturing exchange-repulsion interactions. When compared with MP2 f , the new MP2 f _hb force field predicts properties of liquid water in much better agreement with experiments. The calculated site-site radial distribution functions are in good agreement with X-ray diffraction data corroborating the argument that point-charge models with only Lennard-Jones repulsion underestimate the nearest neighbor O-O distance. The incorporation of the new short-range repulsion term is argued to be important for any force field model that uses point charges to model electrostatics.
Chemical Physics Letters, 2011
The properties of liquid water obtained from molecular simulations using a density functional the... more The properties of liquid water obtained from molecular simulations using a density functional theory (B3LYP) based force field [O. Akin-Ojo, F. Wang, J. Comput. Chem. 32 453.] with and without an intermolecular dispersion interaction term are compared. Although it is small, the dispersion interaction is found to significantly affect some properties such as the density and the heat of vaporization but not others, for example, the atom-atom radial distribution functions and the infrared spectra. The inclusion of the dispersion interaction in the force field produces an increase in the computed self diffusivity comparable to that resulting from a $5 K rise in temperature.
Chemical Physics, 2005
Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H... more Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H2O are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory (SAPT) at two different levels of intramonomer correlation and the supermolecular approach at three different levels of electron correlation. Both CH4 and H2O are assumed to be rigid molecules with interatomic distances and angles fixed at the average values in the ground-state vibration. A physically motivated analytical expression for each PES has been developed as a sum of site-site functions. The PES of the CH4-H2O dimer has only two symmetry-distinct minima. From the SAPT calculations, the global minimum has an energy of -1.03 kcal/mol at a geometry where H2O is the proton donor, HO-H...CH4, with the O-H-C angle of 165 degrees, while the secondary minimum, with an energy of -0.72 kcal/mol, has CH4 in the role of the proton donor (H3C-H...OH2). We estimated the complete basis set limit of the SAPT interaction energy at the global minimum to be -1.06 kcal/mol. The classical cross second virial coefficient B12(T) has been calculated for the temperature range 298-653 K. Our best results agree well with some experiments, allowing an evaluation of the quality of experimental results.
Chemical Physics, 2003
Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCC... more Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCCCN) are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory and the supermolecular method at different levels of electron correlation. HCCCN is taken to be a rigid linear molecule with the interatomic distances fixed at the experimental ``r0'' geometry extracted from ground-state rotational constants. The complex
Chemical Physics, 2003
Potential energy surface and rovibrational spectrum of HeNO dimer. [The Journal of Chemical Phys... more Potential energy surface and rovibrational spectrum of HeNO dimer. [The Journal of Chemical Physics 119, 11654 (2003)]. Ben T. Chang, Omololu Akin-Ojo, Robert Bukowski, Krzysztof Szalewicz. Abstract. Symmetry-adapted ...
Journal of Physical Chemistry A, 2007
Two nonadditive three-body analytic potentials for helium were obtained: one based on three-body ... more Two nonadditive three-body analytic potentials for helium were obtained: one based on three-body symmetry-adapted perturbation theory (SAPT) and the other one on supermolecular coupled-cluster theory with single, double, and noniterative triple excitations [CCSD(T)]. Large basis sets were used, up to the quintuple-zeta doubly augmented size. The fitting functions contain an exponentially decaying component describing the short-range interactions and damped inverse powers expansions for the third- and fourth-order dispersion contributions. The SAPT and CCSD(T) potentials are very close to each other. The largest uncertainty of the potentials comes from the truncation of the level of theory and can be estimated to be about 10 mK or 10% at trimer's minimum configuration. The relative uncertainties for other configurations are also expected to be about 10% except for regions where the nonadditive contribution crosses zero. Such uncertainties are of the same order of magnitude as the current uncertainties of the two-body part of the potential.
Chemical Physics, 2006
We present our calculations of the cross second virial coefficient (B12) and of a related quantit... more We present our calculations of the cross second virial coefficient (B12) and of a related quantity, phi 12 = B12-TdB12/dT, for the methane-water system in the temperature range T = 200-1000 K. These calculations were performed using one of the ab initio potentials developed in previous work [Akin-Ojo and Szalewicz, J. Chem. Phys. 123, 134311 (2005)]. Quantum corrections of order variant Planck's over 2pi(2) were added to the computed classical values. We have estimated the uncertainties in our computed B12 and phi 12(T). This allowed evaluation of the quality of the experimental data to which we compare our results. We also provide an analytical expression for B12(T) as a function of the temperature T obtained by fitting the calculated values. This formula also predicts values of phi12(T) consistent with the directly calculated values.
Chemical Physics, 2011
The second virial coefficient, B12(T), of the H2O-H2 system has been calculated ab initio over th... more The second virial coefficient, B12(T), of the H2O-H2 system has been calculated ab initio over the temperature range 200-700 K. A semi-classical method was employed with two recent accurate potential energy surfaces. The agreement with experimental data is good, although experimental error bars are much larger than the theoretical uncertainties. We show that highly correlated potentials are required for an
This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using ... more This work is an investigation of the spectroscopy and thermodynamics of solvated molecules using cyanoacetylene in helium and methane in water as our two main case studies. For the former system, information on the spectra was obtained, while the latter was used as a prototype to study the hydrophobic effect. The most important elements needed in the study of these systems, namely, the intermolecular potential energy surfaces (PESs) were developed from first principles, i.e., without the use of any experimental data. In the case of the CH4-H2O system, the PESs were also used to calculate the cross second virial coefficient, useful in the natural-gas industry. The predictions of the spectra of the He-HCCCN dimer made in this work have now been confirmed by experiments. By extending the results for this dimer using a simple model, the reduction in the rotational constant of HCCCN upon solvation in a superfluid helium nanodroplet was explained and the value of this constant was predicted. The work on the hydrophobic effect involved molecular dynamics simulations of methane-water mixtures. In some of these calculations, a polarization model was used to go beyond the popularly-used pairwise additivity approximation of the interaction energies for many-body systems. The results from the molecular simulations increase our understanding of the nature of the hydrophobic effect by describing the influence of temperature on this phenomenon and also shedding light on the relation between the hydrophobic effect and the molecular properties of the solute molecules. The need to use the Helmholtz free energy and not simply one of its components (the entropic one as is often done) to explain the aggregation of apolar molecules in water was also emphasized.
Chemical Physics, 2010
The force matching method is used to improve density functional theory (DFT) by designing a suppl... more The force matching method is used to improve density functional theory (DFT) by designing a supplemental potential to capture the difference in atomic forces between a DFT functional and a high-quality post Hartree-Fock method. The supplemental potential has two-body terms designed to correct for dispersion and hydrogen bond interactions. The potential also has one-body terms to improve the description of the intramolecular potential energy surface. Our procedure is tested by providing corrections to the Becke-Lee-Yang-Parr exchange-correlation functional for water and is found to perform significantly better than the standard DFT-D approach, giving QCISD quality predictions for relative cluster energies, atomic forces, and molecular structures. It is found that a simple Lennard-Jones term does a good job at correcting for van der Waals interactions and possibly also providing corrections to exchange repulsion. The one-body corrections, while contributing only slightly to improving relative cluster energies, significantly reduce the errors in binding energies and atomic forces for the systems studied.
Molecular Simulation, 2011
This article summarises the adaptive force matching (AFM) method recently developed by the Wang g... more This article summarises the adaptive force matching (AFM) method recently developed by the Wang group. The AFM method is capable of parameterising force fields in the condensed phase by fitting electronic structure forces obtained through quantum mechanics/molecular mechanics (QM/MM) calculations. The AFM method utilises an iterative procedure to ensure good sampling and high-quality MM models for QM/MM calculations. By fitting
Chemical Physics, 2008
A new method called adaptive force matching (AFM) has been developed that is capable of producing... more A new method called adaptive force matching (AFM) has been developed that is capable of producing high quality force fields for condensed phase simulations. This procedure involves the parametrization of force fields to reproduce ab initio forces obtained from condensed phase quantum-mechanics/molecular-mechanics (QM/MM) calculations. During the procedure, the MM part of the QM/MM is iteratively improved so as to approach ab initio quality. In this work, the AFM method has been tested to parametrize force fields for liquid water so that the resulting force fields reproduce forces calculated using the ab initio MP2 and the Kohn-Sham density functional theory with the Becke-Lee-Yang-Parr (BLYP) and Becke three-parameter LYP (B3LYP) exchange correlation functionals. The AFM force fields generated in this work are very simple to evaluate and are supported by most molecular dynamics (MD) codes. At the same time, the quality of the forces predicted by the AFM force fields rivals that of very expensive ab initio calculations and are found to successfully reproduce many experimental properties. The site-site radial distribution functions (RDFs) obtained from MD simulations using the force field generated from the BLYP functional through AFM compare favorably with the previously published RDFs from Car-Parrinello MD simulations with the same functional. Technical aspects of AFM such as the optimal QM cluster size, optimal basis set, and optimal QM method to be used with the AFM procedure are discussed in this paper.
Journal of Computational Chemistry, 2011
The recently introduced adaptive force matching (AFM) method is used to develop a significantly i... more The recently introduced adaptive force matching (AFM) method is used to develop a significantly improved pair-wise nonpolarizable potential for water. A rigid version of the potential is also presented to enable larger time steps for biological simulations. In this work, it is demonstrated that the AFM method can be used to systematically assess the importance of each functional term during the construction of a force field. For a water potential, it is established that a single off-atom charge center (M) in the plane of water outperforms two out-of-plane charge sites for reproducing intermolecular forces. The four-site pair-wise nonpolarizable force field developed in this work rivals some of the most sophisticated polarizable models in terms of reproducing accurate ab initio forces. The force fields are parameterized to perform best in the temperature range from 0 to 40°C. Equilibrium and dynamical properties calculated with the flexible and rigid force fields are in good agreement with experimental results. For the flexible model, the agreement improves when path integral simulation is performed. These force fields provide high-quality results at a very low computational cost and are thus well suited to atomistic scale biological simulations. The AFM method provides a mechanism for selecting important terms in force field expressions and is a very promising tool for producing accurate force fields in condensed phases.
Journal of Physical Chemistry B, 2009
The MP2 f water force field, recently developed using the adaptive force matching method, is impr... more The MP2 f water force field, recently developed using the adaptive force matching method, is improved significantly after the introduction of a short-range repulsion term. This term is designed to address the deficiency of point-charge models at short charge-charge separations. It may also provide a more realistic description of hydrogen bonds by capturing exchange-repulsion interactions. When compared with MP2 f , the new MP2 f _hb force field predicts properties of liquid water in much better agreement with experiments. The calculated site-site radial distribution functions are in good agreement with X-ray diffraction data corroborating the argument that point-charge models with only Lennard-Jones repulsion underestimate the nearest neighbor O-O distance. The incorporation of the new short-range repulsion term is argued to be important for any force field model that uses point charges to model electrostatics.
Chemical Physics Letters, 2011
The properties of liquid water obtained from molecular simulations using a density functional the... more The properties of liquid water obtained from molecular simulations using a density functional theory (B3LYP) based force field [O. Akin-Ojo, F. Wang, J. Comput. Chem. 32 453.] with and without an intermolecular dispersion interaction term are compared. Although it is small, the dispersion interaction is found to significantly affect some properties such as the density and the heat of vaporization but not others, for example, the atom-atom radial distribution functions and the infrared spectra. The inclusion of the dispersion interaction in the force field produces an increase in the computed self diffusivity comparable to that resulting from a $5 K rise in temperature.
Chemical Physics, 2005
Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H... more Six-dimensional intermolecular potential energy surfaces (PESs) for the interaction of CH4 with H2O are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory (SAPT) at two different levels of intramonomer correlation and the supermolecular approach at three different levels of electron correlation. Both CH4 and H2O are assumed to be rigid molecules with interatomic distances and angles fixed at the average values in the ground-state vibration. A physically motivated analytical expression for each PES has been developed as a sum of site-site functions. The PES of the CH4-H2O dimer has only two symmetry-distinct minima. From the SAPT calculations, the global minimum has an energy of -1.03 kcal/mol at a geometry where H2O is the proton donor, HO-H...CH4, with the O-H-C angle of 165 degrees, while the secondary minimum, with an energy of -0.72 kcal/mol, has CH4 in the role of the proton donor (H3C-H...OH2). We estimated the complete basis set limit of the SAPT interaction energy at the global minimum to be -1.06 kcal/mol. The classical cross second virial coefficient B12(T) has been calculated for the temperature range 298-653 K. Our best results agree well with some experiments, allowing an evaluation of the quality of experimental results.
Chemical Physics, 2003
Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCC... more Five two-dimensional potential energy surfaces for the interaction of He with cyanoacetylene (HCCCN) are presented, obtained from ab initio calculations using symmetry-adapted perturbation theory and the supermolecular method at different levels of electron correlation. HCCCN is taken to be a rigid linear molecule with the interatomic distances fixed at the experimental ``r0'' geometry extracted from ground-state rotational constants. The complex
Chemical Physics, 2003
Potential energy surface and rovibrational spectrum of HeNO dimer. [The Journal of Chemical Phys... more Potential energy surface and rovibrational spectrum of HeNO dimer. [The Journal of Chemical Physics 119, 11654 (2003)]. Ben T. Chang, Omololu Akin-Ojo, Robert Bukowski, Krzysztof Szalewicz. Abstract. Symmetry-adapted ...
Journal of Physical Chemistry A, 2007
Two nonadditive three-body analytic potentials for helium were obtained: one based on three-body ... more Two nonadditive three-body analytic potentials for helium were obtained: one based on three-body symmetry-adapted perturbation theory (SAPT) and the other one on supermolecular coupled-cluster theory with single, double, and noniterative triple excitations [CCSD(T)]. Large basis sets were used, up to the quintuple-zeta doubly augmented size. The fitting functions contain an exponentially decaying component describing the short-range interactions and damped inverse powers expansions for the third- and fourth-order dispersion contributions. The SAPT and CCSD(T) potentials are very close to each other. The largest uncertainty of the potentials comes from the truncation of the level of theory and can be estimated to be about 10 mK or 10% at trimer's minimum configuration. The relative uncertainties for other configurations are also expected to be about 10% except for regions where the nonadditive contribution crosses zero. Such uncertainties are of the same order of magnitude as the current uncertainties of the two-body part of the potential.
Chemical Physics, 2006
We present our calculations of the cross second virial coefficient (B12) and of a related quantit... more We present our calculations of the cross second virial coefficient (B12) and of a related quantity, phi 12 = B12-TdB12/dT, for the methane-water system in the temperature range T = 200-1000 K. These calculations were performed using one of the ab initio potentials developed in previous work [Akin-Ojo and Szalewicz, J. Chem. Phys. 123, 134311 (2005)]. Quantum corrections of order variant Planck's over 2pi(2) were added to the computed classical values. We have estimated the uncertainties in our computed B12 and phi 12(T). This allowed evaluation of the quality of the experimental data to which we compare our results. We also provide an analytical expression for B12(T) as a function of the temperature T obtained by fitting the calculated values. This formula also predicts values of phi12(T) consistent with the directly calculated values.