where we have used ∂ q = (1/e)∂ η . In practice, we use a special case of the Woodbury matrix identity to determine the inverse . The proton occupancy correlator matrix describes the correlations between the protonisation sites on the protein. From Eq. ( ), it follows that proton occupancy fluctuations are anisotropic due to the electrostatic interaction coupling matrix ξ kp [Eq. ( )].