Improving the accuracy of computing chemical potentials in CFCMC simulations

The CFCMC simulation methodology considers an expanded ensemble to solve the problem of low insertion/deletion acceptance probabilities in open ensembles. It allows for a direct calculation of the chemical potential by binning of the coupling parameter λ and using the probabilities p(λ = 0) and p(λ = 1), which require extrapolation. Here, we show that this extrapolation leads to systematic errors when the distribution p(λ) is steep. We propose an alternative binning scheme which improves the accuracy of computed chemical potentials. We also investigate the use of multiple fractional molecules needed in simulations of multiple components, and show that these fractional molecules are very weakly correlated and that calculations of chemical potentials are not affected. The statistics of Boltzmann averages in systems with multiple fractional molecules is shown to be poor. Good agreement is found between CFCMC averages (uncorrected for the bias) and Boltzmann averages when the number of fractional molecules is less than 1% of the total number of all molecules. We found that, in dense systems, biased averages have a smaller uncertainty compared to Boltzmann averages.