Frozen natural orbitals: Systematic basis set truncation for coupled-cluster theory

The method of frozen natural orbital (FNO) basis set truncation for coupled-cluster theory is described. Numerical comparisons of the FNO potential energy surfaces of a group of small molecules at the CCSD(T) level in DZP, cc-pVTZ, cc-pVQZ bases show that truncation of up to 50% of the virtual space yields CC correlation energies that are accurate to 90 or 95% when added to the full MBPT(2) basis result. The FNO truncation method is also applied to dimethylnitramine ( DMNA): both the equilibrium structure and dimer interactions, yielding results at the CCSD( T) level in both a DZP and cc-pVTZ basis set that agree with literature values. CCSD( T) calculations at two possible equilibrium structures of 1,3,5-trinitrohexahydro-1,3,5- triazine (RDX) in a truncated DZP basis are also reported.