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.