Multiconfigurational perturbation theory with level shift — the Cr2 potential revisited

✍ Scribed by Björn O. Roos; Kerstin Andersson

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 623 KB
• Volume: 245
• Category: Article
• ISSN: 0009-2614
• DOI: 10.1016/0009-2614(95)01010-7

✦ Synopsis

A level shift technique is suggested for removal of intruder states in multiconfigurational second-order perturbation theory (CASPT2). The first-order wavefunction is first calculated with a level shift parameter large enough to remove the intruder states. The effect of the level shift on the second-order energy is removed by a back correction technique (the LS correction). It is shown that intruder states are removed with little effect on the remaining part of the correlation energy. I + ~ + New potential curves have been computed for the X ~g and the a' ~, states of Cr 2 using large basis sets (ANO: 8s7p6d4f2g) and accounting for relativistic effects, 3s and 3p correlation, and basis set superposition effects. The computed I + spectroscopic constants (experimental values in parentheses) for the X ~ state are r e = 1.69(1.68) ,~, AGI/2 = 535(452) 3 + = 1.64(1.65) A, AG]/2 =667(574) cm-I, cm -I, Do= 1.54(1.44) eV. The corresponding values for a' E, are r~ T~ = 1.79(I.76) eV.