A new general formula is derived that links the derivatives of the free energy with the instantaneous force acting on selected generalized coordinates. The instantaneous force is defined as the force that, when subtracted from the equations of motion, results in zero acceleration along the coordinate of interest. This formula is applicable to both unconstrained and constrained simulations. For constrained simulations, it reduces to an expression previously derived by den Otter and Brie. The method is tested in two examples: rotation around the C-C bond of 1,2-dichloroethane in water and transfer of fluoromethane across the water-hexane interface. The calculated free energies are compared with those obtained by two commonly used methods: one based on the probability density function and the other on average force in constrained simulations. The results show excellent agreement, and the relative advantages of each method are discussed.A new general formula is derived that links the derivatives of the free energy with the instantaneous force acting on selected generalized coordinates. The instantaneous force is defined as the force that, when subtracted from the equations of motion, results in zero acceleration along the coordinate of interest. This formula is applicable to both unconstrained and constrained simulations. For constrained simulations, it reduces to an expression previously derived by den Otter and Brie. The method is tested in two examples: rotation around the C-C bond of 1,2-dichloroethane in water and transfer of fluoromethane across the water-hexane interface. The calculated free energies are compared with those obtained by two commonly used methods: one based on the probability density function and the other on average force in constrained simulations. The results show excellent agreement, and the relative advantages of each method are discussed.