Rovibrational energy levels of H2O by quantum computing

We calculate rovibrational energy levels of H2O using a trapped-ion quantum computer. We first derive the qubit form of Watson's Hamiltonian, including the rovibrational coupling terms. In a second step, we employ a variant of the quantum-selected configuration-interaction method to calculate rovibrational energy levels. A truncated form of the qubit Hamiltonian is used to generate correlated rovibrational wave functions on the quantum computer by time evolution, and a basis set is selected by sampling from the measured probability distribution. The rovibrational energy levels are obtained by constructing a Hamiltonian matrix using the selected basis set and diagonalizing the matrix using a classical computer. We show that an accuracy of a few hc cm-1 can be achieved for low-lying rovibrational energy levels.