“…However, this method is not applicable in large-scale problems due to the prohibitive computational issue. In order to overcome this limitation, fast iterative methods have been developed such as the Smith method [12], the alternating direction implicit (ADI) method [13], gradient-based methods [14,15], and the Krylov subspace-based algorithm [7,16,17]. At present, the conjugate gradient (CG) method [7] and the preconditioned conjugate gradient method [18] are popularly used with the advantages of small storage and suitability for parallel computing.…”