A general non-Abelian density matrix renormalization group algorithm with application to the C2 dimer

We extend our previous work [S. Sharma and G. K.-L. Chan, J. Chem. Phys. 136, 124121 (2012)], which described a spin-adapted (SU(2) symmetry) density matrix renormalization group algorithm, to additionally utilize general non-Abelian point group symmetries. A key strength of the present formulation is that the requisite tensor operators are not hard-coded for each symmetry group, but are instead generated on the fly using the appropriate Clebsch-Gordan coefficients. This allows our single implementation to easily enable (or disable) any non-Abelian point group symmetry (including SU(2) spin symmetry). We use our implementation to compute the ground state potential energy curve of the C-2 dimer in the cc-pVQZ basis set (with a frozen-core), corresponding to a Hilbert space dimension of 10(12) many-body states. While our calculated energy lies within the 0.3 mE(h) error bound of previous initiator full configuration interaction quantum Monte Carlo and correlation energy extrapolation by intrinsic scaling calculations, our estimated residual error is only 0.01 mE(h), much more accurate than these previous estimates. Due to the additional efficiency afforded by the algorithm, the excitation energies (T-e) of eight lowest lying excited states: a(3)Pi(u), b(3)Sigma(-)(g), A(1)Pi(u), c(3)Sigma(+)(u), B-1 Delta(g), B'(1)Sigma(+)(g), d(3)Pi(g), and C-1 Pi(g) are calculated, which agree with experimentally derived values to better than 0.06 eV. In addition, we also compute the potential energy curves of twelve states: the three lowest levels for each of the irreducible representations (1)Sigma(+)(g), (1)Sigma(+)(u), (1)Sigma(-)(g), and (1)Sigma(-)(u), to an estimated accuracy of 0.1 mE(h) of the exact result in this basis. (C) 2015 AIP Publishing LLC.