  The variational quantum eigensolver is a promising way to solve the
Schr\"odinger equation on a noisy intermediate-scale quantum (NISQ) computer,
while its success relies on a well-designed wavefunction ansatz. Compared to
physically motivated ansatzes, hardware heuristic ansatzes usually lead to a
shallower circuit, but it may still be too deep for an NISQ device. Inspired by
the quantum neural network, we propose a new hardware heuristic ansatz where
the circuit depth can be significantly reduced by introducing ancilla qubits,
which makes a practical simulation of a chemical reaction with more than 20
atoms feasible on a currently available quantum computer. More importantly, the
expressibility of this new ansatz can be improved by increasing either the
depth or the width of the circuit, which makes it adaptable to different
hardware environments. These results open a new avenue to develop practical
applications of quantum computation in the NISQ era.
