title: TD-CI simulation of the electronic optical response of molecules in intense fields: comparison of RPA, CIS, CIS(D), and EOM-CCSD.
abstract: A number of different levels of theory have been tested in TD-CI simulations of the response of butadiene interacting with very short, intense laser pulses. Excitation energies and transition dipoles were calculated with linear-response time-dependent Hartree-Fock (also known as the random phase approximation, RPA), configuration interaction in the space of single excitations (CIS), perturbative corrections to CIS involving double excitations [CIS(D)], and the equation-of-motion coupled-cluster (EOM-CC) method using the 6-31G(d,p) basis set augmented with n = 0-3 sets of diffuse sp functions on all carbons and only on the end carbons [6-31 n+ G(d,p) and 6-31(n+)G(d,p), respectively]. Diffuse functions are particularly important for transitions between the pseudocontinuum states above the ionization threshold. Simulations were carried out with a three-cycle Gaussian pulse (ω = 0.06 au, 760 nm) with intensities up to 1.26 × 10(14) W cm(-2) directed along the vector connecting the end carbons. Depending on the basis set, up to 500 excited states were needed for the simulations. Under the conditions selected, the response was too weak with the 6-31G(d,p) basis set, and the difference between levels of theory was more pronounced. When two or three set of diffuse functions were included on all of the carbons, the RPA, CIS, and EOM-CC results were comparable, but the CIS(D) response was too large compared to the more accurate EOM-CC calculations. Even though the frequency of the pulse is not resonant with any of the ground-to-excited transitions, excitations to valence and pseudocontinuum states occur readily above a threshold in the intensity.
authors.affiliation: Department of Chemistry, Wayne State University, Detroit, Michigan 48202, United States.
authors.forename: Jason A
authors.initials: JA
authors.lastname: Sonk
authors.forename: Marco
authors.initials: M
authors.lastname: Caricato
authors.forename: H Bernhard
authors.initials: HB
authors.lastname: Schlegel
mesh_terms: D003486:Cyanides / Q000737:chemistry; D004583:Electrons; D008094:Lithium / Q000737:chemistry; D056004:Molecular Dynamics Simulation; D011789:Quantum Theory; D013997:Time Factors
chemical_list: D003486:Cyanides; D008094:Lithium