  Motivated by a similar approach for Born-Oppenheimer molecular dynamics, this
paper proposes an extended "shadow" Lagrangian density for quantum states of
superfluids. The extended Lagrangian contains an additional field variable that
is forced to follow the wave function of the quantum state through a rapidly
oscillating extended harmonic oscillator. By considering the adiabatic limit
for large frequencies of the harmonic oscillator, we can derive the two
equations of motions, a Schr\"odinger-type equation for the quantum state and a
wave equation for the extended field variable. The equations are coupled in a
nonlinear way, but each equation individually is linear with respect to the
variable that it defines. The computational advantage of this new system is
that it can be easily discretized using linear time stepping methods, where we
propose to use a Crank-Nicolson-type approach for the Schr\"odinger equation
and an extended leapfrog scheme for the wave equation. Furthermore, the
difference between the quantum state and the extended field variable defines a
consistency error that should go to zero if the frequency tends to infinity. By
coupling the time-step size in our discretization to the frequency of the
harmonic oscillator we can extract an easily computable consistency error
indicator that can be used to estimate the numerical error without any
additional costs. The findings are illustrated in numerical experiments.
