A METHOD OF COMBINED QUANTUM MECHANICAL (QM) MOLECULAR MECHANICS (MM) TREATMENT OF LARGE POLYATOMIC SYSTEMS WITH CHARGE TRANSFER BETWEEN THE QM AND MM FRAGMENTS

摘要

For large molecular systems where neither quantum mechanical (QM) nor molecular mechanics (MM) calculations, applied separately, can solve the problem, we worked out a method of combined QM/MM calculations with an electronically transparent interface between the quantum and classical fragments. Three necessary conditions of (1) fragmentation, (2) interfragment self-consistency, and (3) QM-MM continuity are formulated and satisfied by (a) cutting the system on a 2s2p atom (border atom) that participates with its hybridized orbitals in both fragments and does not serve as a pi bridge between them, (b) introducing an intermediate fragment which is treated by both QM (electronic structure) and MM (geometry optimization), and (c) using a special iterative procedure of double (intrafragment and interfragment) self-consistent (DSC) calculations which realizes the electronically transparent interface and charge transfers between the fragments. The method is implemented in a package of computer programs based on INDO/1 for QM calculations, improved IEH method (ICONC) for the DSC procedure, and SYBYL for the MM treatment, all packages modified accordingly. The calculation of a large molecular system-iron picket-fence porphyrin-yielded results in good agreement with the X-ray determined structure. The charge transfers between the fragments are significant, confirming the importance of electronic transparency of the QM/MM interface. The similar to 0.4 Angstrom out-of-plane position of the iron atom with respect to the porphyrin ring is a quantum effect which cannot be reproduced by pure MM treatment. (C) 1997 John Wiley & Sons, Inc. [References: 44]