Car-Parrinello molecular dynamics on excited state surfaces

This paper describes a method to do ab initio molecular dynamics in electro nically excited systems within the random phase approximation (RPA). Using a dynamical variational treatment of the RPA frequency, which corresponds t o the electronic excitation energy of the system, we derive coupled equatio ns of motion for the RPA amplitudes, the single particle orbitals, and the nuclear coordinates. These equations scale linearly with basis size and can be implemented with only a single holonomic constraint. Test calculations on a model two level system give exact agreement with analytical results. F urthermore, we examined the computational efficiency of the method by model ing the excited state dynamics of a one-dimensional polyene lattice. Our re sults indicate that the present method offers a considerable decrease in co mputational effort over a straight-forward configuration interaction (singl es) plus gradient calculation performed at each nuclear configuration. (C) 1999 American Institute of Physics. [S0021-9606(99)01413-0].