G2(MP2,SVP) theory - saving some more time

G2(MP2,SVP) theory is an even cheaper variant of G2(MP2) with again slightly reduced predictive power.
The major difference to G2(MP2) consists in performing the QCISD(T) and MP4 calculations with a smaller
basis set (split valence plus polarization, SVP). The G2(MP2,SVP) energy at 0 degree Kelvin
E₀(G2MP2SVP) is defined as:

E₀(G2MP2SVP) = E[QCISD(T,FC)/6-31G(d)//MP2(FULL)/6-31G(d)]
        + DE(+3df,2p)
        + DE(HLC)
        + ZPE

The definition of the components being:

DE(+3df,2p) = E[MP2(FC)/6-311+G(3df,2p)//MP2(FULL)/6-31G(d)] - E[MP2(FC)/6-31G(d)//MP2(FULL)/6-31G(d)]

DE(HLC) = -An(beta) - Bn(alpha)
                     A = 5.13 mHartrees; B = 0.19 mHartrees
                     n(alpha) = No. of alpha valence electrons
                     n(beta) = No. of beta valence electrons

ZPE = 0.8929 * ZPE[HF/6-31G(d)]

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

• Optimization and frequency calculation at the HF/6-31G(d) level of theory
• Optimization at the MP2(FULL)/6-31G(d) level of theory
• QCISD(T,FC)/6-31G(d)//MP2(FULL)/6-31G(d) single point
• MP2(FC)/6-311+G(3df,2p)//MP2(FULL)/6-31G(d) single point

Comments:

• Open shell systems are treated using unrestricted wavefunctions (UHF, UMP2 . . )
• The higher level correction (HLC) is included to compensate for remaining deficiencies
  of the method. The B value has been chosen such that the energy of the hydrogen atom
  is exact. The A value has been chosen such as to minimize the error for the predicted
  heat of formation of 55 known molecules.
• The mean absolute deviation for the original G2 neutral set (125 reaction energies) is
  1.63 kcal/mol.The mean absolute deviation for the extended G2 neutral set
  (148 reaction energies) is 1.98 kcal/mol.

Literature:

• L. A. Curtiss, K. Raghavachari, G. W. Trucks, J. A. Pople,
  "Gaussian-2 theory for molecular energies of first- and second-row compounds"
  J. Chem. Phys. 1991, 94, 7221 - 7230.
• L. A. Curtiss, J. E. Carpenter, K. Raghavachari, J. A. Pople
  "Validity of additivity approximations used in Gaussian-2 theory"
  J. Chem. Phys. 1992, 96, 9030 - 9034.
• L. A. Curtiss, K. Raghavachari, J. A. Pople,
  "Gaussian-2 theory using reduced Møller-Plesset orders"
  J. Chem. Phys. 1993, 98, 1293 - 1298.
• M. Glukhovtsev, A. Pross, M. McGrath, L. Radom,
  "Extension of Gaussian-2 (G2) theory to bromine- and iodine-containing molecules:
  Use of effective core potentials"
  J. Chem. Phys. 1995, 103, 1878.
• L. A. Curtiss, P. C. Redfern, B. J. Smith, L. Radom,
  "Gaussian-2 (G2) theory: Reduced basis set requirements"
  J. Chem. Phys. 1996, 104, 5148 - 5152.

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