UCC(4) code. Matches Cfour.
Reference: (1) Bartlett, R. J.; Ii, J.; Kucharski, S. A. ALTERNATIVE COUPLED-CLUSTER ANSATZE II. THE UNITARY COUPLED-CLUSTER METHOD. Chem. Phys. Lett. 1988, 155 (1), 133–140.
Requires:
- Psi4 (https://github.com/psi4/psi4/)
- Numpy
- opt_einsum (https://github.com/dgasmith/opt_einsum)