0 alpha/beta electrons

[joannaqw]

When construct LUCJ operator with 0 beta electron, I ran into the following error:

    operator = ffsim.UCJOpSpinUnbalanced.from_t_amplitudes(t2, n_reps=n_reps, t1=t1)
  File "/.../ffsim/variational/ucj_spin_unbalanced.py", line 517, in from_t_amplitudes
    linalg.double_factorized_t2_alpha_beta(t2ab, tol=tol)
  File "/.../ffsim/linalg/double_factorized_decomposition.py", line 652, in double_factorized_t2_alpha_beta
    t2_mat = t2_amplitudes[
IndexError: arrays used as indices must be of integer (or boolean) type  

relevant lines:

nocc_a, nocc_b, nvrt_a, nvrt_b = t2_amplitudes.shape (line 643)

occ_b, vrt_b = np.meshgrid(range(nocc_b), range(nvrt_b), indexing="ij") (line 647)

in this 0 beta electron case, nocc_b = 0 so occ_b, vrt_b is empty, and t2_amplitdues has an empty array.

potential solution: if the input array is empty --> turn an appropriately shaped set of diag_coulomb_mats, orbital_rotations.

[joannaqw]

adding minimal working example for one fragment of Cr4: one Cr atom: (11o, (6,0))

import numpy as np   
from pyscf import gto, scf, mcscf, ao2mo, tools, fci, cc
import ffsim

nelec_a = 6
nelec_b = 0
norb = 11
hcore = np.load('cr_h1.npy')
eri = np.load('cr_h2.npy')

mol_MO = gto.M()
mol_MO.nelectron = nelec_a + nelec_b
mol_MO.spin = nelec_a - nelec_b 
mol_MO.nao = norb
mf_as_MO= mol_MO.RHF()
mf_as_MO.get_hcore = lambda *args: hcore
mf_as_MO.get_ovlp = lambda *args: np.eye(norb)
mf_as_MO._eri = eri
mf_as_MO.kernel() 
h0e_MO = mf_as_MO.mol.energy_nuc()
mc  = cc.CCSD(mf_as_MO)
mc.kernel()
t1 = mc.t1
t2 = mc.t2
print('t1',t1)
print('t2',t2)

operator = ffsim.UCJOpSpinUnbalanced.from_t_amplitudes(t2, n_reps=2, t1=t1)

[joannaqw]

duplicate issue - see #341

note: updated to python 3.11.9, ffsim 0.0.49 and scipy 1.15 (scipy 1.13 and below will cause error)