def contract_two_to_two(self, subscripts, two, out=None, factor=1.0, clear=True):
"""Contract self with a two-index to obtain a two-index.
**Arguments:**
subscripts
Any of ``abcd,bd->ac`` (direct), ``abcd,cb->ad`` (exchange)
two
The input two-index object. (DenseTwoIndex)
**Optional arguments:**
out, factor, clear
See :py:meth:`DenseLinalgFactory.einsum`
"""
check_options('subscripts', subscripts, 'abcd,bd->ac', 'abcd,cb->ad')
if out is None:
out = DenseTwoIndex(self.nbasis)
if clear:
out.clear()
else:
check_type('out', out, DenseTwoIndex)
if subscripts == 'abcd,bd->ac':
tmp = np.tensordot(self._array2, two._array, axes=([(1,2),(1,0)]))
out._array[:] += factor*np.tensordot(self._array, tmp, [0,0])
elif subscripts == 'abcd,cb->ad':
tmp = np.tensordot(self._array2, two._array, axes=([1,1]))
out._array[:] += factor*np.tensordot(self._array, tmp, ([0,2],[0,2]))
return out
def slice_to_two(self, subscripts, out=None, factor=1.0, clear=True):
"""Returns a two-index contraction of the four-index object.
**Arguments:**
subscripts
Any of ``aabb->ab``, ``abab->ab``, ``abba->ab``
**Optional arguments:**
out, factor, clear
See :py:meth:`DenseLinalgFactory.einsum`
"""
# Error checking
check_options('subscripts', subscripts, 'aabb->ab', 'abab->ab', 'abba->ab')
# Handle output argument
if out is None:
out = DenseTwoIndex(self.nbasis)
else:
check_type('out', out, DenseTwoIndex)
if clear:
out.clear()
# Actual computation
if subscripts == 'aabb->ab':
out._array[:] += factor*np.einsum('xab,xab->ab', self._array, self._array2)
elif subscripts == 'abab->ab':
out._array[:] += factor*np.einsum('xaa,xbb->ab', self._array, self._array2)
elif subscripts == 'abba->ab':
out._array[:] += factor*np.einsum('xab,xba->ab', self._array, self._array2)
return out