  An adaptive BDF2 implicit time-stepping method is analyzed for the phase
field crystal model. The suggested method is proved to preserve a modified
energy dissipation law at the discrete levels if the time-step ratios
$r_k:=\tau_k/\tau_{k-1}<3.561$, a recent zero-stability restriction of
variable-step BDF2 scheme for ordinary differential problems. By using the
discrete orthogonal convolution kernels and the corresponding convolution
inequalities, an optimal $L^2$ norm error estimate is established under the
weak step-ratio restriction $0<r_k<3.561$ ensuring the energy stability. This
is the first time such error estimate is theoretically proved for a nonlinear
parabolic equation. On the basis of ample tests on random time meshes, a useful
adaptive time-stepping strategy is suggested to efficiently capture the
multi-scale behaviors and to accelerate the numerical simulations.
