Renormalized Davidson correction

A slightly improved form of this equation is the renormalized Davidson correction, which is also called the Brueckner correction ... 

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

If the renormalization of the wave function is also taken into account, the (1 - al) quantity is divided by ai, and the corresponding correction is called the renormalized Davidson correction. The effect of higher order excitations is thus estimated from the correlation energy obtained at the CISD level times a factor that measures how important the single-determinant reference is at the CISD level. The Davidson correction does not yield zero for two-electron systems, where CISD is equivalent to full Cl, and it is likely that it overestimates the higher order corrections for systems with few electrons. More complicated correction schemes have also been proposed, but are rarely used. 

Dynamical correlation can be added to the CASSCF reference as to single-reference HF. MCQDPT2 is one variety of second-order perturbation theory, SOCI is analogous to CISD, and SOCI + Q includes the renormalized Davidson correction [104-106] for higher... 

Reaction coordinate, 296, 314, 365, 368 Reaction Path (RP) methods, 390 Reaction surface, 390 Reaction volume, 390 Redundant variables, 34, 327 Relaxation time, in simulations, 380 Renormalized Davidson correction, 137 Resonance, resonance structures, 200 Response, wave function, 242 Restricted Active Space Self-Consistent Field (RASSCF) method, 119 Restricted Hartree-Fock (RHF) method, 70 Restricted Open-shell Hartree-Fock (ROHF) method, 70... 

This correction approximately accounts for the effects of unlinked quadruple excitations (i.e. simultaneous pairs of double excitations), and there are many similar expressions in use. For instance, the renormalized Davidson correction112 is... 

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