A CRITICAL-APPRAISAL OF ELECTROSTATIC CHARGE MODELS FOR MOLECULES

The conventional electrostatic charge models (PD-AC) are constructed s o as to reproduce the molecular electrostatic potential (MESP) on and beyond the van der Waals' (vdW) surface. The MESP distribution has rec ently [S R Gadre, S A Kulkarni and I H Shrivastava (1992) J. Chem. Phy s. 96 5253] been shown to exhibit rich topographical features. With th is in view, a detailed topographical comparison of the MESP derived fr om the charge models, with the respective ab initio (MO) ones is taken up for water, hydrogen sulphide, methane and benzene molecules as tes t cases. It is shown that the point charge models have a fundamental l acuna, viz. they fail to mimic the essential topographical features of MESP. A new model incorporating a small number of floating spherical Gaussians is shown to restore all the critical features of the molecul es under study. A comparative study of the standard deviations of MESP derived from charge models on scaled vdW surfaces further reveals tha t the present model leads to a better representation of ab initio MESP .