TRANS-STILBENE IN METHANOL SOLUTION - ON THE EFFECT OF FLEXIBLE ATOMIC CHARGES IN COMPUTER-SIMULATIONS

The effect of using 'geometry-adapted' atomic charges in computer simu lations is studied by carrying out molecular dynamics simulations of t rans-stilbene (tS) in methanol solution. The atomic charges on tS have been calculated at the Hartree-Fock/6-31G(d, p) level according to th e Merz-Singh-Kollman scheme as a function of the two torsional angles phi(1) and phi(2) across the phenyl groups and ethylenic carbons (C-ph -C-e), fitted to analytical functions q(i)(phi(1), phi(2)) and added t o the force field for calculation of the Coulombic interactions. In sp ite of the non-polar character of tS, the quantum mechanically calcula ted atomic charges in the linkage region are observed to vary more tha n 0.1 \e\ with respect to the changes of the torsional angles phi(1) a nd phi(2). Compared to reference molecular dynamics simulations, carri ed out simultaneously with conventional 'fixed' charges, there is a cl ear effect on the solvation structure when 'flexible' atomic charges o n tS are used. It is expected that the effect of geometry-adapted char ges could be significant in polar, highly flexible molecular systems, such as proteins in solution.