On the Calculation of Atomic Forces in Classical Simulation Using the Charge-on-Spring Method To Explicitly Treat Electronic Polarization

An expression for the atomic forces in simulations using the charge-on-spring (COS) polarizable model is rederived. In previous implementations of COS-based models, contributions arising from the dependence of the induced dipoles (i.e., the positions of the charges-on-spring) on the coordinates of the other sites in the system were not taken into account. However, from calculations on gas-phase dimers we found a significant contribution of these terms. Errors in the forces when neglecting these contributions in condensed-phase calculations can be significantly reduced by choosing an appropriately large value for the size of the charge-on-spring.