  Variational quantum eigensolver (VQE) for electronic structure calculations
is believed to be one major potential application of near term quantum
computing. Among all proposed VQE algorithms, the unitary coupled cluster
singles and doubles excitations (UCCSD) VQE ansatz has achieved high accuracy
and received a lot of research interest. However, the UCCSD VQE based on
fermionic excitations needs extra terms for the parity when using Jordan-Wigner
transformation. Here we introduce a new VQE ansatz based on the particle
preserving exchange gate to achieve qubit excitations. The proposed VQE ansatz
has gate complexity up-bounded to $O(n^4)$ where $n$ is the number of qubits of
the Hamiltonian. Numerical results of simple molecular systems such as BeH$_2$,
H$_2$O, N$_2$, H$_4$ and H$_6$ using the proposed VQE ansatz gives very
accurate results within errors about $10^{-3}$ Hartree.
