Quantum Equation-of-Motion Method with Single, Double, and Triple Excitations

The quantum equation-of-motion (qEOM) method with singles and doubles has been suggested to study electronically excited states while it fails to predict the excitation energies dominated by double excitations. In this work, we present an efficient implementation of the qEOM method with single, double and triple excitations. In order to reduce the computational complexity, we utilize the point group symmetry and perturbation theory to screen triple excitation operators, and the scaling is reduced from Nₒ⁶Nᵥ⁶ to Nₒ⁵Nᵥ⁵. Furthermore, we introduce a perturbation correction to the excitation energy to account for the effect of ignored triple excitation operators. We apply this method to study challenging cases, for which the qEOM-SD method exhibits large errors, such as the 2 ¹ excited state of CH^+ and the 2 ¹ state of H₈ molecule. Our new method yields the energy errors less than 0. 18 eV.