Excitation energies, oscillator strengths, and frequency dependent polarizabilities of Be: Comparison of TDHF, EOM (second order), and MCTDHF

Abstract

AbstractLow‐lying excitation energies from the ground state of Be were calculated using a basis set of 61 Cartesian Gaussian functions. Three approximations were employed: the time‐dependent Hartree–Fock (TDHF), second‐order equations‐of‐motion (EOM), and multiconfigurational time‐dependent Hartree–Fock (MCTDHF). The TDHF excitation energies are 0.5–1.1 eV lower than experiment, and the EOM values are 0.3–1.2 eV lower than experiment, whereas the MCTDHF excitation energies deviate on the absolute average from experiment by only 0.03 eV. We found that in an MCTDHF calculation, any proper MCSCF stationary point is a good reference (i.e., initial) state, not just the ground state. Experimental values for oscillator strength are accurately known only for the 2s2X1S → 2s2p1P0 transition. The TDHF value and the MCTDHF value agree with experiment, but the EOM value does not. The agreement of the TDHF value with experiment seems to be coincidental, because for higher lying transitions the TDHF values differ by approximately a factor of two or more from the more accurate MCTDHF. Frequency independent polarizabilities, α(0), were also calculated with the TDHF, HRPA, and MCTDHF and frequency dependent polarizabilities, β(ω), were calculated with the MCTDHF. No experimental data for Be polarizabilities exist, but we expect the MCTDHF values to be among the most accurate calculations available.