###
CC2 excitation energy calculations on large molecules using
the resolution of the identity approximation

Christof Hättig and Florian Weigend

*
Universität Karlsruhe (TH),
Institut für Pysikalische Chemie,
Lehrstuhl für Theoretische Chemie,
Kaiserstrasse 12, D-76128 Karlsruhe,
Germany
*

*J. Chem. Phys. ***113**, 5154-5161 (2000).

(Received 10 April 2000; accepted 5 Juny 2000)

A new implementation of the approximate coupled cluster
singles and doubles method CC2 is reported, which is
suitable for large scale integral-direct calculations.
It employs the resolution of the identity (RI)
approximation for two-electron integrals to reduce the CPU time
needed for calculation and I/O of these integrals.
We use a partitioned form of the CC2 equations which eliminates
the need to store double excitation cluster amplitudes.
In combination with the RI approximation this formulation of the
CC2 equations leads to a reduced scaling of
memory and disc space requirements with the number of
correlated electrons (*n*) and basis functions
(*N*) to, respectively, O(*N*^{2}) and
O(*n N*^{2}),
compared to O(*n*^{2} N^{2})
in previous implementations.
The reduced CPU, memory and disc space requirements make
it possible to perform CC2 calculations with accurate basis sets
on large molecules, which would not be accessible with
conventional implementations of the CC2 method.
We present an application to vertical
excitation energies of alkenes
C_{2n}H_{2n+2}, for *n* = 1 – 12,
and report results for the lowest lying dipole-allowed
transitions for the TZVPP basis sets,
which for *n* = 12 contain 1108 basis functions.
Comparison with conventional CC2 results
for the smaller alkenes show that for CC2 ground state
energies and for excitation energies for valence states,
the error due to the RI approximation is negligible
compared to the usual basis set error, if auxiliary
basis sets are used which have been optimized for
MP2 energy calculations.

**View Article**
PDF
(access restricted to domain theochem.rub.de)

#### Related articles:

Back to the list of Publications by the Quantum Chemistry (Hättig) Group