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Excitation energies, oscillator strengths, and frequency dependent polarizabilities of Be: Comparison of TDHF , EOM (second order), and MCTDHF
Author(s) -
Graham Richard L.,
Yeager Danny L.
Publication year - 1987
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560310112
Subject(s) - excitation , atomic physics , ground state , gaussian , chemistry , basis (linear algebra) , physics , quantum mechanics , computational chemistry , mathematics , geometry
Low‐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 2 s 2 X 1 S → 2 s 2 p 1 P 0 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.