Davidson correction method

It is possible to make a method approximately size-extensive by adding a correction to the final energy. This has been most widely used for correcting CISD energies. This is a valuable technique because a simple energy correction formula is easier to work with than full Cl calculations, which require an immense amount of computational resources. The most widely used correction is the Davidson correction ... 

Another method for making a method size-extensive is called the self-consistent dressing of the determinant energies. This is a technique for modifying the Cl superdeterminant in order to make a size-extensive limited Cl. The accuracy of this technique is generally comparable to the Davidson correction. It performs better than the Davidson correction for calculations in which the HF wave function has a low weight in the Cl expansion. 

Renormalized Davidson correction, 137 Small Curvature Semiclassical Adiabatic Thermodynamical cycle, 382 Variable metric optimization method, 321... 

Clearly the fraction of the correlation energy recovered by a truncated Cl will diminish as the size of the system increases, making it a progressively less accurate method. There have been many attempts to correct the Cl energy to make it size extensive. The most widely-used (and simplest) of these methods is referred to as the Davidson correction,110,111 which is... 

Paldus, elsewhere in this book, discusses that there is as yet no generally applicable, open-shell, size-extensive, coupled cluster method, and the same holds for open-shell S APT methods. Therefore, for the computation of potentials of open-shell van der Waals molecules one has the choice between CASSCF followed by a Davidson-corrected MRCl calculation of the interaction energy, or the single reference, high spin, method RCCSD(T). When the ground state of the open-shell monomer is indeed a high spin state, then RCCSD(T) is the method of choice. With regard to the latter method we recall that a major difficulty in open-shell systems is the adaptation of the wave function to the total spin operator S for the CCSD method a partial spin adaptation was published by Knowles et al. [219,220] who refer to their method as partially spin restricted . When non-iterative triple corrections [221] are included, the spin restricted CCSD(T) method, RCCSD(T), is obtained. 

In Table I we compare the full Cl correlation energies with those obtained from single-reference type treatments which aim for size consistency, such as the Davidson corrected CI(SD) (and the CI(SD) itself, of course), the MBPT(2) and MBPT(4), ° the CCSD, the symmetry-adapted cluster methods S AC-A and SAC-B and the CPF methods. (We have not included MR-CI(SD)... 

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