Excited states of molecular and crystalline acetylene: application of TDHF and BSE via density fitting methods

Abstract

Low-lying states in small, multiply bonded hydrocarbon molecules may be divided into valence and Rydberg excitations. Excited states of the cubic phase of the acetylene molecular crystal are calculated using the time-dependent Hartree–Fock (TDHF) method. TDHF and Bethe–Salpeter equation (BSE) calculations are performed on molecular CH in order to interpret these excited states. Computationally inexpensive TDHF and BSE methods are compared to previous CASPT2, MRCI and EOM-CCSD calculations and experiment. Localised, lowest energy excited states in the CH molecular crystal mirror those in the gas phase. In the next lowest excitations the electron and hole separate over one or two shells of molecular neighbours. A density fitting method for calculating the Coulomb matrix elements which arise in time-dependent Hartree Fock theory (TDHF) in periodic systems is implemented in the Exciton code and described briefly.