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1   // ******************************************************************************
2   //
3   // Title:       Force Field X.
4   // Description: Force Field X - Software for Molecular Biophysics.
5   // Copyright:   Copyright (c) Michael J. Schnieders 2001-2024.
6   //
7   // This file is part of Force Field X.
8   //
9   // Force Field X is free software; you can redistribute it and/or modify it
10  // under the terms of the GNU General Public License version 3 as published by
11  // the Free Software Foundation.
12  //
13  // Force Field X is distributed in the hope that it will be useful, but WITHOUT
14  // ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
15  // FOR A PARTICULAR PURPOSE. See the GNU General Public License for more
16  // details.
17  //
18  // You should have received a copy of the GNU General Public License along with
19  // Force Field X; if not, write to the Free Software Foundation, Inc., 59 Temple
20  // Place, Suite 330, Boston, MA 02111-1307 USA
21  //
22  // Linking this library statically or dynamically with other modules is making a
23  // combined work based on this library. Thus, the terms and conditions of the
24  // GNU General Public License cover the whole combination.
25  //
26  // As a special exception, the copyright holders of this library give you
27  // permission to link this library with independent modules to produce an
28  // executable, regardless of the license terms of these independent modules, and
29  // to copy and distribute the resulting executable under terms of your choice,
30  // provided that you also meet, for each linked independent module, the terms
31  // and conditions of the license of that module. An independent module is a
32  // module which is not derived from or based on this library. If you modify this
33  // library, you may extend this exception to your version of the library, but
34  // you are not obligated to do so. If you do not wish to do so, delete this
35  // exception statement from your version.
36  //
37  // ******************************************************************************
38  package ffx.potential.bonded;
39  
40  /**
41   * The LambdaInterface should be implemented by potential energy terms that can accept a lambda
42   * value from [0 .. 1] that defines a twice differentiable path between states 0 and 1.
43   *
44   * @author Michael J. Schnieders
45   * @since 1.0
46   */
47  public interface LambdaInterface {
48  
49    /**
50     * Returns true if dUdL is guaranteed to be zero at 0 and 1. Default implementation is to return
51     * false.
52     *
53     * @return True if dUdL is guaranteed 0 at endpoints.
54     */
55    default boolean dEdLZeroAtEnds() {
56      return false;
57    }
58  
59    /**
60     * Get the current value of the state variable.
61     *
62     * @return state
63     * @since 1.0
64     */
65    double getLambda();
66  
67    /**
68     * Set the current value of the state variable. May be ignored if lambda is not being applied.
69     *
70     * @param lambda a double.
71     * @since 1.0
72     */
73    void setLambda(double lambda);
74  
75    /**
76     * Get the 2nd partial derivative of the energy with respect to lambda.
77     *
78     * @return d2EdL2
79     * @since 1.0
80     */
81    double getd2EdL2();
82  
83    /**
84     * Get the partial derivative of the energy with respect to lambda.
85     *
86     * @return dEdL
87     * @since 1.0
88     */
89    double getdEdL();
90  
91    /**
92     * Get the gradient of dEdL with respect to each parameter.
93     *
94     * @param gradient - A double array of length the number of parameters in the model (commonly 3 *
95     *     number of atoms).
96     * @since 1.0
97     */
98    void getdEdXdL(double[] gradient);
99  }