Use of a New Cluster Ansatz to Treat Strong Relaxation and Correlation Effects: A Direct Method for Energy Differences

Abstract

Abstract:We have presented in this paper a new cluster Ansatz for the wave operator for open-shell and/or quasidegenerate states, which takes care of strong relaxation and correlation effects in a compact and efficient manner. This Ansatz allows contraction among the various cluster operators via spectator orbitals, accompanied by suitable combinatorial factors. Since both the orbital and the correlation relaxations are treated on the same footing, it allows us to develop a very useful direct method for energy differences for open shell states relative to a closed-shell ground state, where the total charge for the two states may differ. We have discussed a new spin-free coupled cluster (CC) based direct method and illustrated its performance by evaluating electron affinity of a neutral doublet radical. We have also indicated how the scope of the theory can be extended to compute the state energies of simple open shell configurations as well. In that case, the CC equations terminate after the quartic power of cluster operators -exactly as in the closed-shell situation, which is not the case for the current methods.

A novel VU-MRCC formalism for the simultaneous treatment of strong relaxation and correlation effects with applications to electron affinity of neutral radicals

“…The aug-cc-pVDZ, aug-ccpVTZ and aug-cc-pVQZ are used with their original contraction schemes. The difference between our previous work [31] and the present work is in the description of basis sets, the earlier work never included the diffuse 'f' function in the aug-cc-pVTZ. The results have quite improved, moving from 1.7628 eV [31] to 1.8513 eV, showing that the orbitals of higher angular momentum are rather effective, and the results of our new method match very well with the experimental value.…”

“…The difference between our previous work [31] and the present work is in the description of basis sets, the earlier work never included the diffuse 'f' function in the aug-cc-pVTZ. The results have quite improved, moving from 1.7628 eV [31] to 1.8513 eV, showing that the orbitals of higher angular momentum are rather effective, and the results of our new method match very well with the experimental value. In addition, we have employed a modified aug-cc-pVQZ basis set with tight 's' and 'p' functions having the same exponents and coefficients as in the aug-cc-pCVQZ basis of the EMSL library [65].…”

“…Very recently, we have communicated some interesting results [31] on the electron affinity of OH with both valence-specific as well as valence-universal methods, as the pilot application of our relaxation theory. Present calculations have some extra correction terms, mentioned in the theory, and we study the effect of using a variety of basis sets as well.…”

“…The challenges for the direct methods are: (a) to provide a balanced description of the common correlation terms; (b) to model the differential correlation in a realistic manner; (c) to provide an adequate description of the orbital relaxation accompanying ionization or excitation; and (d) to work with an orbital basis which is flexible enough to incorporate the effects in (a)-(c). In this paper, we want to present the essentials of a recently developed VU-MRCC formalism for treating strong correlation and relaxation effects [30][31][32] and apply it to the computation of the electron affinity (EA) of neutral doublet radicals where orbital relaxation effects are known to be strong.…”

“…Two attractive features of this formalism are its explicitly spin-free nature and the terminating series structure of the working equations for the valence cluster amplitudes. For more extensive presentation of the formulation, we refer to our earlier papers [30][31][32] where the efficacy of the formalism was explored and validated on the core-, inner-and outer-valence ionization potentials. Prompted by the success of the formalism to give excellent results for ionization potential in all the three above energy ranges, it seems worthwhile to study the power and utility of the same method to predict electron affinity of neutral radicals where the orbital relaxation upon anion formation is also very strong.…”