Berny RFO / Rust Berny Backend

RBerny Backend

The RBerny backend is a new implementation of the Berny RFO algorithm. Select it either via the RBerny task token or Algorithm rberny in the OPT section.

task: RBERNY RHF STO-3G

GEOMETRY
  O   0.000000   0.000000   0.100000
  H   0.750000   0.000000  -0.450000
  H  -0.750000   0.000000  -0.450000
END

Backend-specific controls (Dihedral, SuperWeakDih, EnergyNoise) are configured in the OPT section alongside Algorithm rberny.

Dihedral — Control torsional coordinates

By default (Dihedral true) the Rust Berny engine includes dihedral (torsion) angles in its primitive internal‑coordinate set. Setting Dihedral false omits them entirely, which reduces the coordinate count and removes torsional coupling from the initial Hessian guess.

task: RBERNY RHF STO-3G

OPT
  Algorithm  rberny
  Dihedral   false
END

GEOMETRY
  O   0.000000   0.000000   0.100000
  H   0.750000   0.000000  -0.450000
  H  -0.750000   0.000000  -0.450000
END

SuperWeakDih — Soften dihedral Hessian guess

When SuperWeakDih true is set, dihedral angles that would normally receive a "weak" force‑constant scaling (≈ 0.1) in the Lindh Hessian are instead treated as "superweak" (≈ 0.01). This gives very soft initial torsional force constants, useful for floppy molecules where the first few steps should not over‑correct large dihedral motion.

task: RBERNY RHF STO-3G

OPT
  Algorithm     rberny
  SuperWeakDih  true
END

GEOMETRY
  C   0.000000   0.000000   0.000000
  C   1.540000   0.000000   0.000000
  H   0.000000   1.090000   0.000000
  H   0.000000  -1.090000   0.000000
  H   1.540000   1.090000   0.000000
  H   1.540000  -1.090000   0.000000
END

EnergyNoise — Flat‑PES trust‑radius floor

Energy precision estimate in Hartree. The trust‑radius logic treats predicted energy changes below 10 × EnergyNoise as numerical noise, adjusting the trust radius conservatively instead of growing or shrinking aggressively. Increase this on very flat surfaces where tiny energy differences may be unreliable.

task: RBERNY RHF STO-3G

OPT
  Algorithm    rberny
  EnergyNoise  1e-7
END

GEOMETRY
  O   0.000000   0.000000   0.100000
  H   0.750000   0.000000  -0.450000
  H  -0.750000   0.000000  -0.450000
END

Rust Berny Features

Native Cartesian and internal coordinate paths
High-performance Rust implementation
Backend-specific controls: Dihedral, SuperWeakDih, EnergyNoise
First-cycle Cartesian Hessian injection
Hard constraints via native correction flow

Usage

Via task token:

task: RBERNY RHF STO-3G

OPT
  Dihedral  false
END

Via OPT section:

task: OPT RHF STO-3G

OPT
  Algorithm  rberny
  Dihedral   false
END

Notes

The RBerny backend is implemented with native internal coordinate support.
It provides the same algorithmic features as the legacy Berny optimizer but with better performance and Rust-native code paths.
For TS searches, combine with TransitionState true (or the TS task token).

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