Filtering Functions

void qf_ferm_op_filter_diagonal_terms(QfFermionOperator *op)

Filters out the terms of an operator that are diagonal in the occupation-number basis.

This removes every term from the provided QfFermionOperator that is a product of number operators (\(a^\dagger_i a_i\)). This includes the constant term (a product of zero number operators), single number operators, as well as higher-order products such as \(n_i n_j = a^\dagger_i a^\dagger_j a_j a_i\).

Hint

A major motivation for filtering such terms arises during the time evolution of operators, where such diagonal terms, do not affect the time evolution besides introducing a global phase. Consequently, bitstrings sampled from a time evolved state remain unaffected. Removing them is therefore recommended, for example when preparing an electronic-structure Hamiltonian for the qDRIFT time evolution used by SqDRIFT, as it avoids an otherwise unnecessary sampling overhead.

Caution

The provided operator must be normal-ordered! This is an underlying assumption of the implementation that is not being verified! See qf_ferm_op_normal_ordered() for how to get an operator of that form.

1QfFCIDump *fcidump = qf_fcidump_from_file("molecule.fcidump");
2QfFermionOperator *op = qf_ferm_op_from_fcidump(fcidump);
3
4QfFermionOperator *normal = qf_ferm_op_normal_ordered(op, NULL);
5
6qf_ferm_op_filter_diagonal_terms(normal);

Note

The operator is modified in place. If it tracks group indices (see qf_ferm_op_get_groups()), surviving terms retain their relative grouping but the group indices are reassigned to a contiguous range starting from 0. You must therefore not rely on the specific group index of any term being preserved across a call to this function.

Parameters:

  • op – A pointer to the normal-ordered fermionic operator whose terms are to be filtered (modified in place).