Crate omecp 

OpenMECP - High-Performance Minimum Energy Crossing Point Optimizer

OpenMECP is a Rust implementation of the Harvey et al. algorithm for locating Minimum Energy Crossing Points (MECP) between two potential energy surfaces in quantum chemistry calculations.

Overview

The MECP is the geometry where two electronic states have the same energy but different wavefunctions. This is crucial for understanding:

• Photochemical reactions
• Spin-forbidden processes
• Intersystem crossing
• Conical intersections
• Non-adiabatic dynamics

Algorithm

OpenMECP implements the Harvey et al. algorithm (Chem. Phys. Lett. 1994) with modern optimization strategies. The MECP gradient combines two components:

1. f-vector: Drives energy difference to zero

   f = (E1 - E2) * x_norm

2. g-vector: Minimizes energy perpendicular to gradient difference

   g = f1 - (x_norm · f1) * x_norm

Where x_norm = (f1 - f2) / |f1 - f2| is the normalized gradient difference.

Features

• Multiple Optimizers: BFGS, GDIIS, GEDIIS with automatic switching
• QM Program Support: Gaussian, ORCA, Bagel, XTB, Custom interfaces
• Constraint System: Bond, angle, and fixed atom constraints with Lagrange multipliers
• Advanced Methods: LST interpolation, NEB, PES scans, coordinate driving
• Run Modes: Normal, Read, NoRead, Stable, InterRead modes for various scenarios
• Energy Scanning: 1D and 2D potential energy surface scans
• Restart Capability: Checkpoint system for restarting calculations
• Multi-layer QM/MM: ONIOM support for hybrid calculations

Quick Start

use omecp::parser::parse_input;
use std::path::Path;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // Create configuration from file
    let input_data = parse_input(Path::new("input.inp"))?;
    let config = input_data.config;

    // Run MECP optimization
    // let result = config.run()?;

    // println!("MECP converged at step: {}", result.steps);
    Ok(())
}

Supported QM Programs

Program	Methods	Features
Gaussian	DFT, TD-DFT, MP2, CASSCF	Full feature support, checkpoints
ORCA	DFT, TD-DFT, CASSCF	Energy/gradient files, GBW checkpoints
XTB	GFN2-xTB	Fast semi-empirical calculations
Bagel	CASSCF, MRCI	Advanced quantum chemistry methods
Custom	Any	JSON-configurable interface

Optimizers

BFGS (Broyden-Fletcher-Goldfarb-Shanno)

• Quasi-Newton method with PSB Hessian updates
• Default for first 3 steps
• Good general-purpose optimizer

GDIIS (Geometry-based Direct Inversion in Iterative Subspace)

• Automatically activates after 3 BFGS steps
• 2-3x faster convergence than BFGS
• Stores last 4 geometries and gradients
• Computes error vectors for optimal step direction

GEDIIS (Energy-Informed DIIS)

• Enhanced version of GDIIS using energy information
• Set use_gediis = true to enable (or "blend" for new GDIIS_blend method)
• 2-4x faster convergence than GDIIS for difficult cases
• Better handling of energy-difference minimization

Modules

• config - Configuration structures and parsing
• geometry - Core geometry and state data structures
• parser - Input file parsing
• qm_interface - QM program interfaces
• optimizer - MECP optimization algorithms
• constraints - Geometric constraint system
• io - File I/O utilities
• lst - Linear synchronous transit interpolation
• checkpoint - Restart functionality
• reaction_path - NEB and path optimization
• template_generator - Input file templates
• help - Built-in help system

Input File Format

OpenMECP uses a custom input format with section-based syntax:

*GEOM
C  0.0  0.0  0.0
H  1.0  0.0  0.0
*

*TAIL_a
# Additional keywords for state A
*

*TAIL_b
# Additional keywords for state B
*

program = gaussian
method = B3LYP/6-31G*
nprocs = 4
mem = 4GB
charge = 0
mult_state_a = 1  # or mult_a = 1
mult_state_b = 3  # or mult_b = 3

Examples

See the project repository for complete examples including:

• Basic singlet-triplet MECP
• TD-DFT excited states
• Open-shell singlet calculations
• PES scans with constraints
• LST interpolation
• Coordinate driving
• ONIOM calculations

References

• Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theoret. Chim. Acta 1994, 90, 189-194. DOI: 10.1007/BF01120148

License

MIT License - see LICENSE file for details

Version

0.0.1 (Alpha)

Re-exports

pub use config::Config;

pub use geometry::Geometry;

Modules

checkpoint

Restart functionality Checkpoint system for saving and restarting MECP calculations.

cleanup

Automated file cleanup for quantum chemistry calculations Automated file cleanup for quantum chemistry calculations.

config

Configuration structures and parsing for OpenMECP input files.

constraints

Geometric constraint system for molecular optimization.

gdiis

GDIIS (Geometry Direct Inversion in Iterative Subspace) implementation GDIIS (Geometry Direct Inversion in Iterative Subspace) implementation.

gediis

GEDIIS (Geometry Energy Direct Inversion in Iterative Subspace) implementation GEDIIS (Geometry Energy Direct Inversion in Iterative Subspace) implementation.

geometry

Core geometry and state data structures for molecular representations.

help

Built-in help system Comprehensive help system for MECP

hessian_update

Hessian update methods (BFGS, Bofill, Powell) Hessian update methods for optimization algorithms.

io

File I/O utilities for geometry and checkpoint files.

lst

Linear synchronous transit interpolation Linear Synchronous Transit (LST) interpolation module.

naming

Dynamic file naming based on input file basename Dynamic file naming based on input file basename

optimizer

Optimization algorithms for MECP calculations.

parser

Input file parsing for OpenMECP configuration.

pes_scan

PES scanning functionality PES (Potential Energy Surface) scanning functionality.

qm_interface

Quantum chemistry program interfaces for MECP calculations.

reaction_path

NEB and path optimization Reaction path optimization and coordinate driving methods.

settings

Configuration management system Configuration management for OpenMECP.

template_generator

Input file templates

validation

Run mode validation and compatibility checking Run mode validation and compatibility checking for OpenMECP.