Calculation of Wave-functions with Frozen Orbitals in Mixed Quantum Mechanics/Molecular Mechanics methods. II. Application of the Local Basis Equation.

Ferenczy, György G. (2013) Calculation of Wave-functions with Frozen Orbitals in Mixed Quantum Mechanics/Molecular Mechanics methods. II. Application of the Local Basis Equation. Journal of Computational Chemistry, 34. pp. 862-869. ISSN 0192-8651

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Abstract

The application of the local basis equation1 in mixed quantum mechanics/molecular mechanics (QM/MM) and quantum mechanics/quantum mechanics (QM/QM) meth- ods is investigated. This equation is suitable to derive local basis nonorthogonal orbitals that minimize the energy of the system and it exhibits good convergence properties in a self-consistent field solution. These features make the equation appropriate to be used in mixed QM/MM and QM/QM methods to optimize orbitals in the field of frozen localized orbitals connecting the subsystems. Calculations performed for several properties in divers systems show that the method is robust with various choices of the frozen orbitals and frontier atom properties. With appropriate basis set assignment it gives results equivalent with those of a related approach2 using the Huzinaga equation. Thus the local basis equation can be used in mixed QM/MM methods with small size quantum subsystems to calculate properties in good agreement with reference Hartree- Fock-Roothaan results. It is shown that bond charges are not necessary when the local basis equation is applied, although they are required for the self-consistent field solution of the Huzinaga equation based method. On the other hand, the deformation of the wave-function near to the boundary is observed without bond charges and this has a significant effect on deprotonation energies but a less pronounced effect when the total charge of the system is conserved. The local basis equation can also be used to define a two layer quantum system with nonorthogonal localized orbitals surrounding the central delocalized quantum subsystem.

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