Quartic scaling analytical gradients of scaled opposite-spin CC2

Nina O. C. Wintera) and Christof Hättigb)
1 Lehrstuhl für Theoretische Chemie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Chem. Phys. 401, 217-227 (2012).
(Received 16 June 2011; accepted 5 October 2011)

An implementation of analytical gradients, transition moments and excited state properties for scaled opposite-spin (SOS) CC2 and a SOS variant of the algebraic diagrammatic construction through second order ADC(2) is presented. The time-determining fifth order scaling steps in the algorithms are replaced by schemes with 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 for geometry optimizations of large systems. This work is an extension to the recently presented quartic scaling algorithm for SOS-CC2 energies for ground and excited states. The Laplace error for adiabatic electronic excitation energies and excited state structures is found to be very small. SOS-ADC(2) provides for adiabatic electronic excitation energies, excited state structure parameters, harmonic vibrational frequencies as well as dipole moments and transition strengths a similar accuracy than SOS-CC2. Timings for ground and excited state calculations are reported and applications to a chlorophyll molecule and a 2,2':6',2''-bis(terpyridine) with 160 atoms demonstrate how the new implementation extends the applicability of these methods for large systems.

a) Electronic mail: Nina.Winter@theochem.rub.de
b) Electronic mail: christof.haettig@rub.de

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