  We explore the use of symmetry-adapted perturbation theory (SAPT) as a simple
and efficient means to compute interaction energies between large molecular
systems with a hybrid method combing NISQ-era quantum and classical computers.
From the one- and two-particle reduced density matrices of the monomer
wavefunctions obtained by the variational quantum eigensolver (VQE), we compute
SAPT contributions to the interaction energy [SAPT(VQE)]. At first order, this
energy yields the electrostatic and exchange contributions for non-covalently
bound systems. We empirically find from ideal statevector simulations that the
SAPT(VQE) interaction energy components display orders of magnitude lower
absolute errors than the corresponding VQE total energies. Therefore, even with
coarsely optimized low-depth VQE wavefunctions, we still obtain sub kcal/mol
accuracy in the SAPT interaction energies. In SAPT(VQE), the quantum
requirements, such as qubit count and circuit depth, are lowered by performing
computations on the separate molecular systems. Furthermore, active spaces
allow for large systems containing thousands of orbitals to be reduced to a
small enough orbital set to perform the quantum portions of the computations.
We benchmark SAPT(VQE) (with the VQE component simulated by ideal state-vector
simulators) against a handful of small multi-reference dimer systems and the
iron center containing human cancer-relevant protein lysine-specific
demethylase 5 (KDM5A).
