  We present a numerical implementation of the time-dependent surface flux
(tSURFF) method [New J. Phys. 14, 013021 (2012)], an efficient computational
scheme to extract photoelectron energy spectra, to the time-dependent
multiconfiguration self-consistent-field (TD-MCSCF) method. Extending the
original tSURFF method developed for single particle systems, we formulate the
equations of motion for the spectral amplitude of orbital functions
constutiting the TD-MCSCF wave function, from which the angle-resolved
photoelectron energy spectrum, and more generally, photoelectron reduced
density matrices (RDMs) are readiliy obtained. The tSURFF method applied to the
TD-MCSCF wave function, in combination with an efficient absorbing boundary
offered by the infinite-range exterior complex scaling, enables accurate {\it
ab initio} computations of photoelectron energy spectra from multielectron
systems subject to an intense and ultrashort laser pulse with a computational
cost significantly reduced compared to that required in projecting the total
wave function onto scattering states. We apply the present implementation to
the photoionization of Ne exposed to an attosecond extreme-ultraviolet (XUV)
pulse and above-threshold ionization of Ar irradiated by an intense
mid-infrared laser field, demonstrating both accuracy and efficiency of the
present method.
