Quantum Monte Carlo Study of the Protonated Water Dimer

We report an extensive theoretical study of the protonated water dimer H5O2+ (Zundel ion) by means of the highly correlated variational Monte Carlo and lattice regularized Monte Carlo approaches. This system represents the simplest model for proton transfer (PT), and a correct description of its properties is essential in order to understand the PT mechanism in more complex aqueous systems. Our Jastrow correlated AGP wave function ensures an accurate treatment of electron correlation. By exploiting the advantage of contracting the primitive basis set over atomic hybrid orbitals, we are able to limit dramatically the number of variational parameters with a systematic control on the numerical precision, a crucial ingredient in order to simulate larger systems. For both energetics and geometrical properties, our QMC results are found to be in excellent agreement with state-of-the-art coupled cluster CCSD(T) techniques. A comparison with density functional theory in the PBE approximation points to the crucial role of electron correlation for a correct description of the PT in the dimer. We prove that the QMC framework used in this work is able to resolve the tiny energy differences (similar to 0.3 kcal/mol) and structural variations involved in proton transfer reactions. Our approach combines these features and a favorable N-4 scaling with the number of particles which paves the way to the simulation of more realistic PT models. A test calculation on a larger protonated water cluster is carried out. The QMC approach used here represents a promising candidate to provide the first high-level ab initio description of PT in water.