CCSD -coupled clusters

For all intents and purposes then, we are concerned here with the CCSD (coupled cluster with all single and double substitutions [30]) correlation energy. Its convergence is excruciatingly slow Schwartz [31] showed as early as 1963 that the increments of successive angular momenta l to the second-order correlation energy of helium-like atoms converge as... 

The single-reference coupled cluster (CC) theory [1-5] has become a standard computational tool for studying ground-state molecular properties [6-10]. The basic approximations, such as CCSD (coupled cluster singles and doubles approach) [11-15], and the noniterative CCSD[T] [16,17] and CCSD(T) [18] methods, in which the cleverly designed corrections due to... 

CCSD Coupled cluster with single and double substitution operators... 

CCSD coupled-cluster theory with single and double excitation terms... 

CCSD Coupled cluster theory including singe and double excitations... 

CCSD Coupled-cluster with singles and doubles... 

CCSD Coupled Cluster including Single and Double excitations. 

CCSD coupled cluster with single and ciouble excitation... 

CASPT2 complete aetive spaee based second-order perturbation theory CCSD coupled-cluster single and double CCSD(T) CCSD corrected with triple excitations Cl configmation interaction... 

AIMD = ab initio molecular dynamics B-LYP = Becke-Lee-Yang-Parr CCSD = coupled cluster single double excitations CVC = core-valence correlation ECP = effective core potential DF = density functional GDA = gradient corrected density approximation MCLR = multiconfigurational linear response MP2 = M0ller-Plesset second-order (MRD)CI = multi-reference double-excitation configuration interaction RPA = random phase approximation TD-MCSCF = time-dependent multiconfigurational self-consistent field TD-SCF = time-dependent self-consistent field. 

Alternatively, there are perturbation methods to estimate Ecorreiation- Briefly, in these methods, you take the HF wavefunction and add a correction—a perturbation—that better mimics a multi-body problem. Moller-Plesset theory is a common perturbative approach. It is called MP2 when perturbations up to second order are considered, MP3 for third order, MP4, etc. MP2 calculations are commonly used. Like CISD, MP2 allows single and double excitations, but the effects of their inclusion are evaluated using second-order perturbation theory rather than variationally as in CISD. An even more accurate type of perturbation theory is called coupled-cluster theory. CCSD (coupled-cluster theory, singles and doubles) includes single and double excitations, but their effects are evaluated at a much higher level of perturbation theory than in an MP2 calculation. 

B-CC = Brueckner CC CCD = coupled-cluster doubles CCSD = coupled-cluster singles and doubles CCSDT = coupled-cluster singles, doubles, and triples CCSDTQ = coupled-cluster singles, doubles, triples, and quadruples Cl = configuration interaction EOM = equation-on-motion FCI = full-configuration interaction. 

CCSD = coupled-cluster with singles and doubles CCSD(T) = CCSD with noniterative triple excitations Cl = configuration interaction CISD = Cl singles and doubles CP = counterpoise correction QCISD = quadratic Cl with singles and doubles. 

ATM = Axilrod-Teller-Muto term CCD = coupled-clu.s-ter doubles CCSD = coupled-cluster singles and doubles CP = counterpoise (technique) DCBS = dimer-centered basis set HS = Hirschfelder-Silbey theory SAPT = symmetry-adapted perturbation theory SRS = symmetrized Rayleigh-Schrd-dinger series. 

CISD... Configuration Interaction. Single and Double Excitation. CCSD... Coupled Cluster. Single and Double Excitation. 

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