Contents: Table of Contents   Inactive core and virtual  Chapter 3  CISD, CEPA, CCD, QCISD and QCISD(T) calculations

Brueckner coupled cluster calculations

Coupled cluster (CC) is one of the most accurate ab initio methods. It is commonly truncated at the single and double excitations level (CCSD). It is possible to carry out a CCSD calculation in which the effects of the single excitations are absorbed into the orbitals. The resulting orbitals are called Brueckner orbitals, and a determinant composed of Brueckner orbitals has maximum overlap with the exact wavefunction possible for one determinant. This type of calculation is called the Brueckner Doubles method (BD) and it is, in principle, more accurate than a conventional CCSD calculation. The program will also carry out BD calculations with an additional perturbative estimate of the effects of triple excitations - BD(T).

Here is an example dataset.

TITLE
H2O Simple example of BD (Brueckner Doubles)
SYMMETRY
CNV 2
END
BASIS 631G*
ATOMS
 O   8     0.0000000000        0.0000000000        0.6579994716
 H   1    -1.4554305563        0.0000000000        1.9632324335
END
BD
ENERGY
START
FINISH

The calculation proceeds by an iterative scheme (for each iteration the output is a more elaborate version of that seen in the QCISD example mentioned earlier) and when finished gives the correlation energy and the total energy. In the example given above this is,

 FINAL VALUE OF THE CORRELATION ENERGY IS  -0.205534884793198
 FINAL VALUE OF THE TOTAL ENERGY IS   -76.2005146917758

The degree of convergence of the iterative process may be controlled by the CICONV keyword - the format is given in the preceding section. For BD(T) calculations simply replace BD in the above dataset by BD(T).