  Simulations with an adaptive time-dependent bias, such as metadynamics,
enable an efficient exploration of the conformational space of a system.
However, the dynamic information of the system is altered by the bias. With
infrequent metadynamics it is possible to recover the transition rate of
crossing a barrier, if the collective variables are ideal and there is no bias
deposition near the transition state. Unfortunately, for simulations of complex
molecules, these conditions are not always fulfilled. To overcome these
limitations, and inspired by single-molecule force spectroscopy, we developed a
method based on Kramers' theory for calculating the barrier-crossing rate when
a time-dependent bias is added to the system. We assess the quality of the bias
parameter by measuring how efficiently the bias accelerates the transitions
compared to ideal behavior. We present approximate analytical expressions of
the survival probability that accurately reproduce the barrier-crossing time
statistics, and enable the extraction of the unbiased transition rate even for
challenging cases, where previous methods fail.
