The dependency of hydration free energies on the atomic charge distrib
ution has been studied using the free energy perturbation method and m
olecular dynamics simulations. Several hypothetical sets of atomic cha
rges far methanal molecule have been constructed, yielding the same to
tal molecular dipole moment of 2.14 D. Each of the sets has been pertu
rbed into another one and the electrostatic contributions to the hydra
tion free energy differences associated with such changes have been de
termined. The magnitude of calculated electrostatic contribution to th
e solvation free energies crucially depends an the position of the cen
ter of charge and orientation of the total dipole moment within molecu
lar van der Waals envelope. It depends less on higher molecular multip
ole moments. The issue of convergence of the electrostatic contributio
n to the free energies obtained from molecular dynamics simulations wi
ll be also addressed.