Article pubs.acs.org/JCTC
First UHF Implementation of the Incremental Scheme for Open-Shell Systems Tony Anacker,† David P. Tew,‡ and Joachim Friedrich*,† †
Institute for Chemistry, Chemnitz University of Technology, Straße der Nationen 62, D-09111 Chemnitz, Sachsen, Germany School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom
‡
S Supporting Information *
ABSTRACT: The incremental scheme makes it possible to compute CCSD(T) correlation energies to high accuracy for large systems. We present the first extension of this fully automated black-box approach to open-shell systems using an Unrestricted Hartree−Fock (UHF) wave function, extending the efficient domain-specific basis set approach to handle open-shell references. We test our approach on a set of organic and metal organic structures and molecular clusters and demonstrate standard deviations from canonical CCSD(T) values of only 1.35 kJ/mol using a triple ζ basis set. We find that the incremental scheme is significantly more cost-effective than the canonical implementation even for relatively small systems and that the ease of parallelization makes it possible to perform high-level calculations on large systems in a few hours on inexpensive computers. We show that the approximations that make our approach widely applicable are significantly smaller than both the basis set incompleteness error and the intrinsic error of the CCSD(T) method, and we further demonstrate that incremental energies can be reliably used in extrapolation schemes to obtain near complete basis set limit CCSD(T) reaction energies for large systems.
1. INTRODUCTION The ability to compute electronic energies to high accuracy for large systems is essential for modern science, and a substantial effort is being directed into making systematically improvable methods like coupled-cluster theory applicable to even larger systems. The primary goal is to compute coupled-cluster singles, doubles, and perturbative triples [CCSD(T)] energies, which, in combination with an appropriate basis set, is currently the ”gold standard” of quantum chemistry.1−3 However, traditionally this method has been limited to small- and medium-sized molecules with
