A multistate local coupled cluster CC2 response method based on the Laplace transform.

A new Laplace transform based multistate local CC2 response method for calculating excitation energies of extended molecular systems is presented. By virtue of the Laplace transform trick, the eigenvalue problem involving the local CC2 Jacobian is partitioned along the doubles-doubles block (which is diagonal in the parental canonical method) without losing the sparsity in the integral, amplitude, and amplitude response supermatrices. Hence, only an effective eigenvalue problem involving singles vectors has to be solved, while the doubles part can be computed on-the-fly. Within this framework, a multistate treatment of excited states with state specific and adaptive local approximations imposed on the doubles part is straightforwardly possible. Furthermore, in the context of the density fitting approximation of the two-electron integrals, a procedure to specify the local approximation, i.e., the restricted pair lists and domains, on the basis of an analysis of the object to be approximated itself is proposed. Performance and accuracy of the new Laplace transformed density fitted local CC2 (LT-DF-LCC2) response method are tested for set of different test molecules and states. It turns out that LT-DF-LCC2 response is much more robust than the earlier local CC2 response method proposed before, which failed to find some excited states in difficult cases.

Comment on "Minimax approximation for the decomposition of energy denominators in Laplace-transformed Møller-Plesset perturbation theories" [J. Chem. Phys. 129, 044112 (2008)].

The recently proposed minimax quadrature for Laplace-transformed energy denominators in second order Møller-Plesset perturbation theory is compared to the conventionally used least-squares quadrature. Our test calculations show that the quality of the least-squares quadrature is not inferior compared to minimax. Its computational overhead (required to determine points and weights of the quadrature) is clearly insignificant relative to the subsequent MP2 calculation. For high accuracy and all-electron calculations (correlated core) good starting guesses for the roots are essential.

On the use of the Laplace transform in local correlation methods.

The applicability of the Laplace transform ansatz of Almlöf in the context of local correlation methods with a priori restricted sets of wavefunction parameters is explored. A new local MP2 method based on the Laplace transform ansatz is described, its relation to the local MP2 method based on the Pulay ansatz is elucidated, and its accuracy and efficiency are compared to the latter.

Transition strengths and first-order properties of excited states from local coupled cluster CC2 response theory with density fitting.

A new ab initio method for calculating transition strengths and orbital-unrelaxed first-order properties of singlet ground and excited states of extended molecular systems is presented. It is based on coupled cluster response theory at the level of the CC2 model with local approximations introduced to the doubles-excitation part of the wave function. Density fitting is employed for the calculation of the electron repulsion integrals, so that--with the exception of doubles amplitudes--only three-indexed objects do occur in the formalism. The new method was tested by performing calculations for a set of various molecules and excited states and by comparing the results with corresponding canonical (nonlocal) calculations. It turned out that for calculating transition strengths and properties of excited states the ordinary Boughton-Pulay domains are insufficient in numerous cases. To circumvent this problem a new scheme for extending domains is proposed, which is based on the solution of the coupled perturbed localization and Hartree-Fock equations. When such extended domains are used, a satisfactory agreement between canonical and local results is achieved.

Local CC2 electronic excitation energies for large molecules with density fitting.

A new local method for the computation of electronic excitation energies of singlet states in extended molecular systems is presented. It is based on the CC2 model and local approximations to the wave functions. In the proposed method the singles excitations are treated nonlocally and local restrictions are imposed on doubles amplitudes only. The accuracy of the new method was tested by calculating several lowest excited states for 14 molecules and comparing them with canonical CC2 values. Deviations of the local excitation energies from the canonical reference values do not exceed 0.05 eV for all test molecules and all states in the lower energy range investigated in this work. The method uses the density-fitting approximation for all two-electron integrals, which considerably simplifies the computational complexity of the individual diagrams. A combination of the local approximations and the powerful density-fitting technique leads to a low-scaling method, capable to treat molecular systems comprised of 100 atoms and more in a basis of a polarized double zeta quality. A test calculation for a system consisting of 127 atoms and 370 active electrons without symmetry is presented to show the efficiency of the new method.