Reference Systems
All free-energy methods are based on calculation of free-energy differences
Example
- volume of R can be measured as a fraction of the total volume
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sample the reference system
keep an average of the fraction of time occupying target system
- what we get is the difference
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Usefulness of free-energy difference
- it may be the quantity of interest anyway
- if reference is simple, its absolute free energy can be evaluated analytically
e.g., ideal gas, harmonic crystal