ToMatrix

M = ResponseFunction.ToMatrix(G) returns a matrix representation of $G$ such that $$ G(\omega,\Gamma) = A_0 + B_0^* \left( \frac{1}{(\omega+\mathrm{i}\Gamma/2) - M} \right)_{[1..Blocksize,1..Blocksize]} B_0^T$$

We only need the left top matrix of dimension Blocksize of the inverse of the matrix $(\omega+\mathrm{i}\Gamma/2) - M$. As a result the matrix $M$ is not uniquely determined. Any unitary transformation of the bath, i.e. all rows and columns with index larger than Blocksize does not change $G$. As a consequence $M$ is not uniquely defined. The exact form of $M$ returned depends on the type used for the response function. See ToTightbinding for more information on the relation between the internal representations of response functions and tight binding Hamiltonians, full matrix representations, or operators.