Previously, apply_driving_force set absolute position to A*cos(2π f t + φ),
ignoring the atom's initial coordinates. For atoms not at the origin (e.g.,
atom 120 at x=119), this incorrectly forced them back toward the origin each
step, causing severe distortion and numerical explosion.
Fix: store each driven atom's initial position as eq_pos/eq_x/eq_y/eq_z at
load time; position is now eq + A*cos(2π f t + φ) in all four engines
(Python, C, C++, Fortran).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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