Thermodynamic integration with harmonic reference

The Helmholtz free energy ($A$) of a fully interacting system (1) can be expressed in terms of that of harmonic system (0) as follows

$A_{1} = A_{0} + \Delta A_{0\rightarrow 1}$

where $\Delta A_{0\rightarrow 1}$ is anharmonic free energy. The latter term can be determined by means of thermodynamic integration

$\Delta A_{0\rightarrow 1} = \int_0^1 d\lambda \langle V_1 -V_0 \rangle_\lambda$

with $V_i$ being the potential energy of system $i$, $\lambda$ is a coupling constant and $\langle\cdots\rangle_\lambda$ is the NVT ensemble average of the system driven by the Hamiltonian

$\mathcal{H}_\lambda = \lambda \mathcal{H}_1 + (1-\lambda)\mathcal{H}_0,$