A QM/MM program using frozen localized orbitals and the Huzinaga equation

Abstract

A mixed QM/MM computer program coupling Amber and MRCC is presented. This is the first implementation of the Huzinaga equation-based local self-consistent field (HLSCF) method that makes it possible to calculate ab initio wave functions without orthogonalizing the basis set to the frozen orbitals separating the QM and MM subsystems. A significant novelty of the program is that it includes an automatic generation of the frozen localized orbitals obtained from calculations performed for model molecules cut out of the system. The Amber–Mrcc code also allows the use of the link atom (LA) approach. Sample calculations were performed to check the performance of both the HLSCF and the LA approaches by describing the interactions between the QM and MM subsystems with electrostatic embedding. It was found that the conceptually appealing HLSCF method is a competitive alternative to the LA method.