The effect of correcting the size-extensivity error in singles and doublesconfiguration interaction excitation energies

The size-consistent self-consistent singles and doubles configuration inter action method (SDCI) has been used to get size-extensive excitation energie s into the framework of the SDCI space. The method is applied through a mat rix dressing operator that usually accounts for the dressing effects carrie d by the doubles. This operator has been modified to take into account the dressing effects of singles too. The new operator is relevant when MOs othe r than the canonical SCF ones are used. The extensivity error correction is analyzed depending on the nature of the MOs, the inclusion or not of singl es dressing, and the single or diexcited nature of the excited states. Exci ted stares of BH, CH+, SiH2 and CH2 are used as FCI reference benchmarks fo r comparison. The addition of the singles correction appears to be chiefly relevant for doubly-excited states. The importance of the extensivity corre ction is evidenced in systems with a larger number of correlated electrons, such as cis-butadiene and cyclopentadiene. (C) 2001 Elsevier Science B.V. All rights reserved.