R. Crespo et al., Electronic excitation in a saturated chain: An MS-CASPT2 treatment of the anti conformer of n-tetrasilane, J PHYS CH A, 104(37), 2000, pp. 8593-8599
The singlet-singlet electronic spectrum of the anti conformer of n-tetrasil
ane has been studied using multiconfigurational wave functions (CASSCF), se
cond-order perturbation theory (CASPT2), and its multistate extension (MS-C
ASPT2), in conjunction with large ANO-type basis sets including Rydberg fun
ctions. The calculations include the 4s, 4p, and 3d members of the Rydberg
series converging on the first ionization. Mixing of valence and Rydberg st
ates observed in the CASSCF wave functions is not fully rectified by single
-reference CASPT2 theory, whereas the MS-CASPT2 method separates the valenc
e and Rydberg states effectively. At the MS-CASPT2 level, six valence excit
ed states have been found below the lowest Rydberg transition, predicted at
7.40 eV. In terms of natural orbitals, they correspond to single electron
promotions from the a-symmetry HOMO to four sigma* and two pi* valence orbi
tals. Vertical dipole-allowed valence transition energies (oscillator stren
gths) are computed at 6.33 eV (1.12), 6.68 eV (0.01), and 6.96 eV (0.15), f
or excitation to 1(1)B(u), 1(1)A(u), and 2(1)B(u), states, respectively. Th
ree vertical dipole-forbidden valence transitions are predicted to be inter
leaved among them at 6.55 eV (2(1)A(g)), 6.87 eV (3(1)A(g)), and 7.10 eV (1
(1)B(g)). The results are consistent with the available experimental data a
nd are easily rationalized in terms of a simple Huckel model of sigma deloc
alization.