  The exact evaluation of the molecular ground state in quantum chemistry
requires an exponentially increasing computational cost. Quantum computation is
a promising way to overcome the exponential problem using polynomial-time
quantum algorithms. A quantum-classical hybrid optimization scheme known as the
variational quantum eigensolver(VQE) is preferred for noisy intermediate-scale
quantum devices. However, the circuit depth becomes one of the bottlenecks of
its application to large molecules of more than 20 qubits. In this work, we
employ the point group symmetry to reduce the number of operators in
constructing ansatz so as to achieve a more compact quantum circuit. We
illustrate this methodology with a series of molecules ranging from LiH(12
qubits) to C2H4(28 qubits). A significant reduction of up to 82% of the
operator numbers is reached on C2H4, which enables the largest molecule ever
numerically simulated by VQE-UCC to the best of our knowledge. This also shed
light into the further work of this direction to construct even shallower
ansatz with enough expressive power and simulate even larger scale system.
