The
stiffness of a system
with respect to a deformation (e.g., the stiffness of a spring with respect
to stretching) is the second derivative of the energy
with respect to the corresponding displacement; this measures the curvature
of the potential energy
surface along a particular direction. Positive stiffness is associated with
stability, and a large stiffness can result in a small positional uncertainty
in the presence of thermal excitation. Negative stiffnesses correspond to unstable locations on the potential energy surface. Alternative terms for stiffness
include force gradient and rigidity.
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