An efficient implementation of the direct random-phase approximation usingthe quasi-particle formalism

An efficient direct integral-driven algorithm for the random-phase approxim ation (RPA) is introduced using the equation of motion on a transition dens ity matrix representing a "quasi particle". In the algorithm, several roots are obtained at the same time by solving a set of coupled equations that a re projected on a space spanned by a set of error vectors representing the quasi particles. The most time-consuming RPA operation on the vectors is ac complished by a single call of an integral-generation routine, and the time per iteration is comparable to that for a direct SCF cycle. The algorithm is implemented using a new integral package based on accompanying coordinat e expansion (ACE), as well as traditional integral routines from GAMESS. Th e example applications indicate good convergence of the iterative scheme. I n some computationally intensive cases, the RPA computation for several exc ited states is completed in less time than the SCF computation.