Protocol for W2 theory

The protocol obtained by introducing the succesful approximations given above will be denoted here as W2 (Weizmann-2) theory. Its steps consist of the following:

• geometry optimization at the CCSD(T)/VQZ+1 level, i.e. CCSD(T)/VQZ if only first-row atoms are present;
• zero-point energy obtained from a CCSD(T)/VTZ+1 anharmonic force field or, failing that, B3LYP/VTZ+1 frequencies scaled by 0.985 (vide infra);
• Carry out CCSD(T)/AVTZ+2d1f and CCSD(T)/AVQZ+2d1f single-point calculations;
• Carry out a CCSD/AV5Z+2d1f single-point calculation;
• the SCF component of TAE is extrapolated by A+B/C^l from SCF/AVTZ+2d1f, SCF/AVQZ+2d1f, and SCF/AV5Z+2d1f results (l=3, 4, and 5, respectively);
• the CCSD valence correlation component is obtained from applying A+B/l³ to CCSD/AVQZ+2d1f; and CCSD/AV5Z+2d1f valence correlation energies (l=4 and 5, respectively). It is immaterial whether this is done on total energies or on components to TAE;
• the (T) valence correlation component is obtained from applying A+B/l³ results to CCSD(T)/AVTZ+2d1f and CCSD(T)/AVQZ+2d1f values for the (T) contribution. It is again immaterial whether this is done on total energies or on components to TAE;
• core correlation computed at CCSD(T)/MTsmall level
• scalar relativistic corrections (and, if necessary, spin-orbit splittings) computed at ACPF/MT level. To save CPU time, this can be combined into a single job with the previous step.

On a typical workstation at the time of writing (e.g. an SGI Octane with 1 GB of RAM and 2$\times$18 GB external disks) its applicability range would be about three heavy atoms in C_2v symmetry, although the main limiting factor would be disk space and larger systems could be treated if a direct CCSD code were available.

We will illustrate the CPU time savings made in the W2 approach, compared to our most rigorous calculations, using two examples: a first-row diatomic (CO) and a second-row molecule (OCS). Using MOLPRO on an SGI Octane workstation, the most accurate calculations reported in this work (Table IV) required 21h36' for CO and no less than 362h12' for OCS. W2 theory yields essentially identical results at a cost of 1h12' (CO) or 13h42'(OCS) -- a reduction by a factor of 20-30 which is typical of the other molecules.