  While quantum algorithms for simulation exhibit better asymptotic scaling
than their classical counterparts, they currently cannot be implemented on
real-world devices. Instead, chemists and computer scientists rely on costly
classical simulations of these quantum algorithms. In particular, the quantum
phase estimation (QPE) algorithm is among several approaches that have
attracted much attention in recent years for its genuine quantum character.
However, it is memory-intensive to simulate and intractable for moderate system
sizes. This paper discusses the performance and applicability of QPESIM, a new
simulation of the QPE algorithm designed to take advantage of modest
computational resources. In particular, we demonstrate the versatility of
QPESIM in simulating various electronic states by examining the ground and
core-level states of H$_2$O. For these states, we also discuss the effect of
the active-space size on the quality of the calculated energies. For the
high-energy core-level states, we demonstrate that new QPE simulations for
active spaces defined by 15 active orbitals significantly reduce the errors in
core-level excitation energies compared to earlier QPE simulations using
smaller active spaces.
