  Time-dependent coupled-cluster method with time-varying orbital functions,
called time-dependent optimized coupled- cluster (TD-OCC) method, is formulated
for multielectron dynamics in an intense laser field. We have successfully
derived equations of motion for CC amplitudes and orthonormal orbital functions
based on the real action functional, and implemented the method including
double excitations (TD-OCCD) and double and triple excitations (TD-OCCDT)
within the optimized active orbitals. The present method is size extensive and
gauge invariant, a polynomial cost-scaling alternative to the time-dependent
multiconfiguration self-consistent-field method. The first application of the
TD-OCC method to intense-laser driven correlated electron dynamics in Ar atom
is reported.
