Instanton rate constant calculations close to and above the crossover temperature

Canonical instanton theory is known to overestimate the rate constant close to a system‐dependent crossover temperature and is inapplicable above that temperature. We compare the accuracy of the reaction rate constants calculated using recent semi‐classical rate expressions to those from canonical instanton theory. We show that rate constants calculated purely from solving the stability matrix for the action in degrees of freedom orthogonal to the instanton path is not applicable at arbitrarily low temperatures and use two methods to overcome this. Furthermore, as a by‐product of the developed methods, we derive a simple correction to canonical instanton theory that can alleviate this known overestimation of rate constants close to the crossover temperature. The combined methods accurately reproduce the rate constants of the canonical theory along the whole temperature range without the spurious overestimation near the crossover temperature. We calculate and compare rate constants on three different reactions: H in the Müller–Brown potential, methylhydroxycarbene → acetaldehyde and H2 + OH → H + H2O. © 2017 Wiley Periodicals, Inc. Overestimation of the rate constant k of an Eckart barrier by instanton theory close to the crossover temperature Tc compared to the analytic solution. The contribution proposes a technique to correct this overestimation which is computationally advantageous to the traditional canonical instanton approach.