Extended Coupled Cluster Techniques for Excited States: Applications to Quasispin Models

New Alternatives for Accurate Electronic Structure Calculations of Potential Energy Surfaces Involving Bond Breaking

“…CC (L, T), eq (36), is a finite series in Τ and Σ* (the expectation value of the Hamiltonian with the CC wave function is a nonterminating series in Τ and 7*). There are several other advantages of using eq (36) and the related equations for Τ and Σ as means to determine the cluster operator 71 For example, the ECC methods (114)(115)(116)(117)(118)(119)(120)(121)(122)(123)(124) and the related XCC approaches (124)(125)(126) are capable of describing various higherorder effects, including the effect of Γ 4 clusters, even at the low levels of approximation, such as ECCD or ECCSD. Those effects are being brought into the ECC formalism as products of the low-order many-body components of Σ 1 " and Τ and their powers.…”

“…One such source might be provided by the ECC theory (114)(115)(116)(117)(118)(119)(120)(121)(122)(123) or by its modified variant described in ref 124. The ECC methods are based on the idea of optimizing the cluster operator Τ along with the auxiliary operator Σ by considering the energy functional ^(Σ, Τ), eq (36).…”

“…The proposed solution, termed the quasi-variational MMCC (QVMMCC) formalism, and its quadratic MMCC (QMMCC) variant, both described in this article, are based on combining the idea of the MMCC noniterative energy corrections with the elements of the extended CC (ECC) (114)(115)(116)(117)(118)(119)(120)(121)(122)(123)(124) and expectation value CC (XCC) (124)(125)(126) theories, in which products involving cluster operators and their deexcitation counterparts are used to mimic the effects of higher-order clusters, such as T A . In designing the QMMCC approach, we were particularly inspired by the recent work by Head-Gordon and coworkers (27,28,(127)(128)(129), who demonstrated that the variational CCD (CC doubles) calculations, based on minimizing the expectation value of the Hamiltonian with the CCD wave function, and the quadratic approximation to the bi-variational ECCD (ECC doubles) method lead to a qualitatively correct description of triple bond breaking in N 2 , eliminating, in particular, the nonvariational collapse of the standard CCD theory at large internuclear separations.…”

On the Bargmann Space Approach to the Extended Coupled Cluster Method for Simple Anharmonic Systems

“…In each case the excited-state energies were calculated by using the dynamical matrix obtained from the second-order derivatives of the energy functional, as explained in an accompanying paper in this volume. 15) In Fig. 1we show the results for the first six eigenvalues corresponding to even- .…”