An optional `fixed` model: other atoms of the crystal held where they are -
the other copies of a molecular-replacement search - whose Fcalc is added to
the body's at every evaluation and whose atoms the bulk-solvent mask covers
together with the body's (computed once per zone, with the body where the
zone starts, and held). With them the origin is no longer free, so the gauge
projection is skipped. On the CPU the fixed part is gridded once per zone; on
the GPU the fixed atoms must be copies of the body, and each one's placement
is read off by Kabsch superposition and composed on the device with the
engine's own density, transform and mask (SetFixedCopies), so a fit beside
three copies of a 90 kDa model costs what a fit alone does.
Without `fixed` nothing changes (the composition kernel's extra term is null).
Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EizBKhTqrAYago9KJG3dA3
leonarski_fandClaude Opus 5.5
dcb01100e7