The use of free energy simulation techniques in the study of protein s
tability is critically evaluated. Results from two simulations of the
thermostability mutation Asn218 to Ser218 in Subtilisin are presented.
It is shown that components of the free energy change can be highly s
ensitive to the computational details of the simulation leading to the
conclusion that free energy calculations cannot currently be used to
reliably predict protein stability. The different factors that undermi
ne the reliability are discussed.