interatomic 0.5.0

// Copyright 2023-2024 Mikael Lund
//
// Licensed under the Apache license, version 2.0 (the "license");
// you may not use this file except in compliance with the license.
// You may obtain a copy of the license at
//
//     http://www.apache.org/licenses/license-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the license is distributed on an "as is" basis,
// without warranties or conditions of any kind, either express or implied.
// See the license for the specific language governing permissions and
// limitations under the license.

//! ## Fourbody interactions
//!
//! Module for describing exactly four particles interacting with each other, such as in a dihedral.

use dyn_clone::DynClone;

pub mod harmonic;
pub mod periodic;
pub mod spline;

pub use harmonic::HarmonicDihedral;
pub use periodic::PeriodicDihedral;
pub use spline::SplinedDihedral;

/// Potential energy between four particles as a function of dihedral angle between them.
pub trait FourbodyAngleEnergy: DynClone {
    /// Interaction energy between four particles.
    fn fourbody_angle_energy(&self, angle: f64) -> f64;

    /// Torque: -dU/dφ. Default uses central difference.
    fn fourbody_angle_force(&self, angle: f64) -> f64 {
        const EPS: f64 = 1e-6;
        -(self.fourbody_angle_energy(angle + EPS) - self.fourbody_angle_energy(angle - EPS))
            / (2.0 * EPS)
    }
}

dyn_clone::clone_trait_object!(FourbodyAngleEnergy);