In mathematics, the folded spectrum method (FSM) is an iterative method for solving large eigenvalue problems. Here you always find a vector with an eigenvalue close to a search-value \( \varepsilon \) . This means you can get a vector Ψ {\displaystyle \Psi } \Psi in the middle of the spectrum without solving the matrix.
\( {\displaystyle \Psi _{i+1}=\Psi _{i}-\alpha (H-\varepsilon \mathbf {1} )^{2}\Psi _{i}} \), with \( {\displaystyle 0<\alpha ^{\,}<1} \)and \( \mathbf {1} \) the Identity matrix.
In contrast to the Conjugate gradient method, here the gradient calculates by twice multiplying matrix \( {\displaystyle H:\;G\sim H\rightarrow G\sim H^{2}.} \)
Literature
MacDonald, J. K. L. (1934-11-01). "On the Modified Ritz Variation Method". Physical Review. American Physical Society (APS). 46 (9): 828–828. doi:10.1103/physrev.46.828. ISSN 0031-899X.
Wang, Lin Wang; Zunger, Alex (1994). "Electronic Structure Pseudopotential Calculations of Large (.apprx.1000 Atoms) Si Quantum Dots". The Journal of Physical Chemistry. American Chemical Society (ACS). 98 (8): 2158–2165. doi:10.1021/j100059a032. ISSN 0022-3654.
Wang, Lin‐Wang; Zunger, Alex (1994). "Solving Schrödinger's equation around a desired energy: Application to silicon quantum dots". The Journal of Chemical Physics. AIP Publishing. 100 (3): 2394–2397. doi:10.1063/1.466486. ISSN 0021-9606.
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