Long-range dispersion forces between molecules subject to ultra-short optical pulses from ab initio calculations

London-van der Waals dispersion forces are a fundamental component of condensed matter systems, biological processes, and self-assembly. In this letter we propose a method to calculate the C6 coefficients that characterize dispersion forces in the non-retarded regime for molecules in a coherent superposition of excited states. Several ultrafast femtosecond pump-probe schemes are investigated. We apply the method to LiH molecules and show that their London-van der Waals interaction can change dramatically after the interaction with the pump pulse. The pulse modulates the C6 coefficients, and the interplay between polarization, orientation of the molecules, and the dipole fields gives rise to a rich variety of combinations.