Parallel to the set based on TI are perturbative methods such as free energy perturbation (FEP) methods. The simplest estimators belonging to this class of perturbative methods are those based on the Zwanzig relation. However, it is known in FEP calculations that free energy estimates from the Zwanzig relation can be prone to bias stemming from the dominant contribution of rare samples when using finite sampling . As such there exists several methods which aim to improve on the exponential averaging estimator; these are the Bennet acceptance ratio (BAR) and the Multistate Bennet acceptance ratio (MBAR). In this work the MBAR estimator is used, the derivation of this method is given in detail in the work of Shirts and Chodera et al. . Here we present the relevant equation from this previous work for computing dimensionless free energies in a system with K total λ states,