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We demonstrate how to extend the pair natural orbital (PNO) methodology for excited states, presented in a previous work for the perturbative doubles correction to configuration interaction singles (CIS(D)), to iterative coupled cluster methods such as the approximate singles and doubles model CC2. The original O(N5) scaling of the PNO construction is reduced by using orbital-specific virtuals (OSVs) as an intermediate step without spoiling the initial accuracy of the PNO method. Furthermore, a slower error convergence for charge-transfer states is analyzed and resolved by a numerical Laplace transformation during the PNO construction, so that an equally accurate treatment of local and charge-transfer excitations is achieved. With state-specific truncated PNO expansions, the eigenvalue problem is solved by combining the Davidson algorithm with deflation to project out roots that have already been determined and an automated refresh with a generation of new PNOs to achieve self-consistency of the PNO space. For a large test set, we found that truncation errors for PNO-CC2 excitation energies are only slightly larger than for PNO-CIS(D). The computational efficiency of PNO-CC2 is demonstrated for a large organic dye, where a reduction of the doubles space by a factor of more than 1000 is obtained compared to the canonical calculation. A compression of the doubles space by a factor 30 is achieved by a unified OSV space only. Moreover, calculations with the still preliminary PNO-CC2 implementation on a series of glycine oligomers revealed an early break even point with a canonical RI-CC2 implementation between 100 and 300 basis functions. [ABSTRACT FROM AUTHOR]

An implementation of scaled opposite-spin CC2 (SOS-CC2) for ground and excited state energies is presented that requires only fourth order scaling computational costs. The SOS-CC2 method yields results with an accuracy comparable to the unscaled method. Furthermore the time-determining fifth order scaling steps in the algorithm can be replaced by only fourth order scaling computational costs using a 'resolution of the identity' approximation for the electron repulsion integrals and a Laplace transformation of the orbital energy denominators. This leads to a significant reduction of computational costs especially for large systems. Timings for ground and excited state calculations are shown and the error of the Laplace transformation is investigated. An application to a chlorophyll molecule with 134 atoms results in a speed-up by a factor of five and demonstrates how the new implementation extends the applicability of the method. A SOS variant of the algebraic diagrammatic construction through second order ADC(2), which arises from a simplification of the SOS-CC2 model, is also presented. The SOS-ADC(2) model is a cost-efficient alternative in particular for future extensions to spectral intensities and excited state structure optimizations. [ABSTRACT FROM AUTHOR]

The Cotton-Mouton effect (magnetic field induced linear birefringence) has been studied for neon and argon using state-of-the-art coupled cluster techniques. The coupled cluster singles, doubles and triples (CCSDT) approach has been used to obtain static benchmark results and the CC3 model with an approximate treatment of triple excitations to obtain frequency-dependent results. In the case of neon the effect of excitations beyond triples has also been estimated via coupled cluster calculations including quadruple excitations (CCSDTQ), pentuple excitations (CCSDTQP), etc. up to the full configuration-interaction level. The results obtained for the anisotropy of the hypermagnetizability Δη(ω), the molecular property that determines the magnetic field induced birefringence of spherically symmetric systems, are Δη=2.89 a.u. for neon and Δη=24.7 a.u. for argon, with a negligible effect of frequency dispersion. For neon we could estimate an absolute error on Δη of 0.1 a.u. The accuracy of these results surpasses that of recently reported experimental data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An implementation of transition moments and excited-state first-order properties is reported for the approximate coupled-cluster singles-and-doubles model (CC2) using the resolution of the identity (RI) approximation. In parallel to the previously reported code for the ground- and excited-state amplitude equations, we utilize a partitioned form of the CC2 equations and thus eliminate the need to store any N[SUP4] intermediates. This opens the perspective for applications on molecules with 30 and more atoms. The accuracy of the RI approximation is tested for a set of 29 molecules for the aug-cc-pVXZ (X = D, T, Q) basis sets in connection with the recently optimized auxiliary basis sets. These auxiliary basis sets are found to be sufficient even for the description of diffuse states. The RI error is compared to the usual basis set error and is demonstrated to be insignificant. [ABSTRACT FROM AUTHOR]

Abstract: The excited-state potential-energy surfaces of the lowest-energy conformer of the tryptophan–glycine dipeptide have been investigated with ab initio electronic-structure methods. The calculations reveal a potentially very efficient excited-state deactivation mechanism via conical intersections of the excited states of the indole chromophore with locally-excited and charge-transfer states of the peptide backbone. These findings suggest that the excited-state lifetime of the lowest-energy conformer of the Trp-Gly dipeptide may be too short to allow the detection of a resonant two-photon ionization signal. It is proposed that the efficient excited-state deactivation enhances the photostability of the dipeptide. [Copyright &y& Elsevier]

4-(N-N-Dimethyl-amino)benzonitrile (DMABN) is a prototype molecule for dual fluorescence. The anomalous emission has been attributed to an intramolecular charge-transfer (ICT) state, and the structure of the latter is still subject to some controversy. We applied a recently developed analytical gradient code for the approximate coupled-cluster singles-and-doubles method CC2 in combination with accurate basis sets to address this problem. Fully optimized excited state structures are presented for the ICT state and the so-called locally excited state, and recent transient IR and Raman measurements on the excited states are interpreted by means of calculated harmonic frequencies. Strong evidence is found for an electronic decoupling of the phenyl and the dimethyl-amino moiety, resulting in a minimum structure for the Id state with a twisted geometry. In contrast to previous findings, the structure of this state is, at least in the gas phase, not C2v symmetric but distorted towards Cs symmetry. The distortion coordinate is a pyramidalization of the phenyl carbon atom carrying the dimethyl-amino group. The results from the CC2 model are supported by single-point calculations using more elaborate coupled-cluster models (CCSD, CCSDR(3)) and by CASSCF calculations. [ABSTRACT FROM AUTHOR]