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Add support for flexible barostats in i-PI - #47
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`_qbaro` and `_pbaro` were transcriptions of `BaroBZP.qcstep`/`pstep` with the scalar algebra baked in, so they only worked for the isotropic (BZP) barostat. An MTK barostat, whose `p` is a 3x3 upper-triangular matrix, passed the integrator type check (i-PI restricts the `nst` ensemble to the RGB barostat, so `flexible` runs with `NPTIntegrator`) and then propagated silently wrong dynamics through broadcasting. Both helpers now take a `mode` argument using i-PI's barostat mode names, with the flexible branch following `BaroMTK` -- matrix exponentials for the cell scaling, the full `np.triu` stress for the cell momentum, and the `hmask`/`vol_constraint` projection with its `ethermo` bookkeeping. As in the isotropic branch, the terms that the plain velocity Verlet splitting needs but `OAbBbBABbAbPO` does not (the `sinh` drift term and the `pc.fc` force corrections) are left out. The mode is derived from `type(motion.barostat)`, so it cannot disagree with the input file, and unsupported barostats (SC-BZP, RGB, dummy) now raise instead of being silently mishandled. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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get_npt_stepperonly worked with isotropic (BZP) barostats, because_qbaroand_pbarohad the logic ofBaroBZPbaked in. An MTK barostat still passed theNPTIntegratortype check.flexibleruns withNPTIntegratorand then silently produced wrong dynamics through broadcasting.We can't call
BaroMTK.qcstep/pstepdirectly. Similar toBaroBZP, they assume a plain velocity Verlet splitting in which the position propagation is interleaved with the barostat steps. FlashMD's splitting does the position propagation entirely inside the ML step, so thesinhdrift term and thepc·fcforce corrections have to be left out. These are the same two omissions the existing isotropic branch already makes compared toBaroBZP. Everything else followsBaroMTK: the matrix exponentials for the cell scaling, the fullnp.triustress and thekb*T*Lterm for the cell momentum, and thehmask/vol_constraintprojection with itsethermobookkeeping.The mode is determined automatically from
type(motion.barostat)instead of being passed in, so it can't disagree with the input file, and unsupported barostats (SC-BZP, RGB, dummy) now raise instead of being mishandled.