Coupled cluster singles and doubles variational quantum eigensolver ansatz for electronic structure calculations
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Abstract
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, first order trotterization UCCSD VQE has gate complexity up-bounded to $O(n^5)$ using Jordan-Wigner transformation, where n is the number of qubits of the Hamiltonian. The high complexity makes UCCSD difficult to be implemented on near term quantum computer. Here we introduce a new VQE ansatz based on the particle preserving exchange gate to achieve excitations. The proposed improved VQE ansatz has gate complexity up-bounded to $O(n^4)$. Numerical results of simple molecular systems such as BeH$_2$, H$_2$O, N$_2$, H$_4$ and H$_6$ using the proposed improved VQE ansatz gives very accurate results within chemical accuracy of about $10^{-3}$ Hartree.