Quantum Mechanics-Molecular Mechanics Model Study of some Antibiotics and Vitamins in Gas Phases: Investigation of Energy and NMR Chemical Shift

Abstract

The combination of Quantum Mechanics (QM) and Molecular Mechanics (MM) methods has<br />become alternative tool for many applications that pure QM and MM could not be suitable.<br />The QM/MM method has been used for different type of problems, for example: structural<br />biology, surface phenomena, and liquid phase. In this paper we have performed these methods for<br />some antibiotics and vitamins and then we compare results. The calculations have done by full ab<br />initio method (HF/3-21g), (11F/6-31G) and (HF/STO-3G) and QM/MM (ONIOM) method with<br />FIF (3-21G)/AM I /UFF IlF(6-31G)/AM I /UFF and HF (STO-3G)/AMI/UFF then we find out the<br />geometry that has obtained by QM/MM method is very accurate and we can use this rapid method<br />in place of time consuming ab initio methods for large molecule. The comparison of energy<br />values in QM/MM and QM methods is given. In the present work we compare chemical shifts<br />and conclude that QM/MM method is a perturbed full QM method.