Q-Chem 4.4 User’s Manual

5.11 Non-Hartree-Fock Orbitals in Correlated Calculations

In cases of problematic open-shell references, e.g., strongly spin-contaminated doublet radicals, one may choose to use DFT orbitals, which can yield significantly improved results [313]. This can be achieved by first doing DFT calculation and then reading the orbitals and turning the Hartree-Fock procedure off. A more convenient way is just to specify EXCHANGE, e.g., EXCHANGE = B3LYP means that B3LYP orbitals will be computed and used in the CCMAN/CCMAN2 module, as in the following example.

Example 5.91  CCSD calculation of triplet methylene using B3LYP orbitals

0 3
H 1 CH
H 1 CH 2 HCH

CH  = 1.07
HCH = 111.0

jobtype            SP            single point
exchange           b3lyp
correlation        ccsd
BASIS              cc-pVDZ