G3(MP2)B3 theory - the most economical method

G3(MP2)B3 is an adaption of G3(MP2) theory based on geometries and zero point vibrational
energies calculated at the Becke3LYP/6-31G(d) level of theory. The G3(MP2)B3 energy at
0 degree Kelvin E₀(G3MP2B3) is defined as:

E₀(G3MP2B3) = E[QCISD(T,FC)/6-31G(d)//B3LYP/6-31G(d)]
        + DE(G3MP2large)
        + DE(HLC)
        + ZPE
        + DE(SO)

The definition of the components being:

DE(G3MP2large) = E[MP2(FC)/G3MP2large//B3LYP/6-31G(d)] - E[MP2(FC)/6-31G(d)//B3LYP/6-31G(d)]

DE(HLC) = -An(beta) - B(n(alpha) - n(beta))
                     A = 10.041 mHartrees; B = 4.995 mHartrees (for molecules)
                     A = 10.188 mHartrees; B = 2.323 mHartrees (for atoms)
                     n(alpha) = No. of alpha valence electrons
                     n(beta) = No. of beta valence electrons

ZPE = 0.960 * ZPE[B3LYP/6-31G(d)]

The necessary energies can be calculated most efficiently in the following sequence:

• Optimization and frequency calculation at the Becke3LYP/6-31G(d) level of theory
• QCISD(T,FC)/6-31G(d)//B3LYP/6-31G(d) single point
• MP2(FC)/G3MP2large//B3LYP/6-31G(d) single point

Comments:

• Open shell systems are treated using unrestricted Kohn-Sham orbitals (UB3LYP).
• The higher level correction (HLC) is supposed to compensate remaining deficiencies
  of the method. In contrast to G2 theory, separate parameters A and B have been optimized
  for atoms and molecules to give the smallest average absolute deviation from experiment.
• Spin orbit correction terms E(SO) (mainly of experimental origin) are added only for atoms.
• The G3MP2large basis set is almost identical to the G3large basis used in G3 except that
  core polarization functions are not included. A local copy of the G3MP2large basis set can be found here.
• The mean absolute devitiation for the extended G2 neutral set (148 reaction energies)
  is 1.13 kcal/mol.

Literature:

• L. A. Curtiss, K. Raghavachari, P. C. Redfern, V. Rassolov, J. A. Pople,
  "Gaussian-3 (G3) theory for molecules containing first and second-row atoms"
  J. Chem. Phys. 1998, 109, 7764 - 7776.
• L. A. Curtiss, K. Raghavachari,
  "G2 Theory"
  The Encyclopedia of Computational Chemistry, P. v. R. Schleyer (editor-in-chief),
  John Wiley & Sons Ltd, Athens, USA, 1998, 2, 1104 - 1114.
• A. G. Baboul, L. A. Curtiss, P. C. Redfern, K. Raghavachari,
  "Gaussian-3 theory using density functional geometries and zero-point energies"
  J. Chem. Phys. 1999, 110, 7650 - 7657.