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 Matrix Product Operators (MPO) for two-body Hamiltonians
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The function to use is directHmpo in mpo_direct.py, where an example can be also found.
It allows to construct MPO for two-body H for any grouping of orbitals (specified by partition of spin orbitals), e.g.,

365    k = 10
366    h1e,h2e = genRandomH(k)
367    partition = [[0,1,2,3],[4,5],[6,7],[8,9]]
368    ngroups,wfacs = directHmpo(h1e,h2e,partition,isym=0)
369    print ngroups
370    for i in range(ngroups):
371       print wfacs[i].shape

The output is a list of W[k] for MPO.

Formula can be found in doc/formula.pdf using defintions in Appendix of the paper:
J. Chem. Phys. 145, 014102 (2016); doi: 10.1063/1.4955108
View online: http://dx.doi.org/10.1063/1.4955108