TY - JOUR
T1 - Ab Initio QM/MM Simulations with a Molecular Orbital-Valence Bond (MOVB) Method
T2 - Application to an SN2 Reaction in Water
AU - Mo, Yirong
AU - Gao, Jiali
PY - 2000/12
Y1 - 2000/12
N2 - A mixed molecular orbital and valence bond (MOVB) method is described in combined ab initio QM/MM simulations of the SN2 reaction of Cl- + CH3Cl → ClCH3 + Cl- in water. The method is based on the construction of individual charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential energy surface in comparison with Hartree-Fock, MP2, and ab initio VB calculations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importance of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with experiment and with previous computational results. Importantly, the MOVB method allows for the solvent reaction coordinate to be used to define the reaction path in ab initio QM/MM simulations.
AB - A mixed molecular orbital and valence bond (MOVB) method is described in combined ab initio QM/MM simulations of the SN2 reaction of Cl- + CH3Cl → ClCH3 + Cl- in water. The method is based on the construction of individual charge-localized, diabatic states using a block-localized wave function approach, followed by configuration interaction calculations to obtain the adiabatic potential energy surface. To examine the performance of the MOVB method, modern ab initio VB calculations were performed. The MOVB gas phase results are found to be in reasonable agreement in the overall potential energy surface in comparison with Hartree-Fock, MP2, and ab initio VB calculations. The only exception is that the activation energy is predicted to be about 4 kcal/mol higher in MOVB than in other methods. This is attributed to the configuration interaction procedure, which does not further optimize orbital coefficients in MOVB calculations, and it emphasizes the importance of orbital optimization in these calculations. The adiabatic ground-state potential surface can also be approximate by other quantum chemical models in Monte Carlo QM/MM simulations. At the HF/6-31G(d) level, the calculated activation free energy of 26 kcal/mol in water is in good agreement with experiment and with previous computational results. Importantly, the MOVB method allows for the solvent reaction coordinate to be used to define the reaction path in ab initio QM/MM simulations.
KW - Combined QM/MM method
KW - Mixed molecular orbital and valence bond method
KW - Monte Carlo simulations of chemical reaction in solution
KW - Solvent effects on S2 reaction
KW - Valence bond theory
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U2 - 10.1002/1096-987X(200012)21:16<1458::AID-JCC4>3.0.CO;2-2
DO - 10.1002/1096-987X(200012)21:16<1458::AID-JCC4>3.0.CO;2-2
M3 - Article
AN - SCOPUS:0000342940
SN - 0192-8651
VL - 21
SP - 1458
EP - 1469
JO - Journal of Computational Chemistry
JF - Journal of Computational Chemistry
IS - 16
ER -