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

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