![]() For example, the partial change of hydrogen is +0.09 e. The partial charge of each atom is defined in the fourth row (purple). For the topology information, covalent bonds are defined in the lines starting with “BOND”, where adjacent atom names (blue or green pairs) are connected by covalent bonds. The mass of each atom is defined in the lines beginning with “MASS” (red). The main information contained in this file is the atom connectivity (topology), mass, and charge of the amino acids.īelow are definitions for the atomic mass and topology of alanine (ALA). Next, let’s take a look at the contents of top_all36_prot.rtf. # Take a look at the parameter file for proteins $ less par_all36m_prot.prm : BONDSĬAI CAI 305.000 1.3750 ! atm, methylindole, fit CCDSSĬPT CA 300.000 1.3600 ! atm, methylindole, fit CCDSSĬPT CAI 300.000 1.3600 ! atm, methylindole, fit CCDSSĬPT CPT 360.000 1.3850 ! atm, methylindole, fit CCDSSįor detailed description of the parameter files, see the CHARMM manual ( parmfile). Let’s find the parameters for other energy terms such as angle, dihedral angle, van der Waals, and so on. In this section, we can find bond force constants (3rd column) and equilibrium bond lengths (4th column) for each atom type or atom pair in the amino acids. The following shows a part of the parameters for the bond energy term. Let’s take a look at the contents of par_all36m_prot.prm. Par_all22_prot.prm stream toppar_water_ions.str Openmm_gbsaobc2 silicates toppar_all.history Gbsw par_all36m_prot.prm top_all36_lipid_ljpme.rtf # Check the contentsĠ0toppar_file_format.txt par_all36_cgenff.prm top_all35_ethers.rtfĪce par_all36_lipid.prm top_all36_carb.rtfĬheq par_all36_lipid_ljpme.prm top_all36_cgenff.rtfĭrude par_all36_na.prm top_all36_lipid.rtf Note that top_all36_prot.rtf is common to both CHARMM C36 and C36m. Thus, for example, par_all36m_prot.prm is related to the CHARMM C36m force field parameters of protein (or amino acid residue) for the all-atom model, while top_all36_prot.rtf is related to the topology information of protein (or amino acid residue) for the all-atom model. “ prot“, “ na“, and “ lipid” mean protein, nucleic acid, and lipid, respectively. rtf files are the “parameter” and “residue topology” files, respectively. So, in this page we would like to introduce some important or frequently used files. There are a lot of files in the toppar_c36_jul21 directory. Toppar toppar_c36_jul21 toppar_c36_jul21.tgz # Move to the directory where save the parameter files Put the downloaded file there and unzip it. Let’s move to the “ Data” directory, and then “ Parameters” directory, in which the the force field parameter files are stored. There might be an important update that improves the accuracy of the MD simulation. Please check this page regularly for your research. This file contains the CHARMM C36m parameters, which is the latest version of the CHARMM force field at this time. Since we are writing this tutorial in December 2021, we would like to select “ toppar_c36_jul21.tgz“. Let’s go to the Mackerell’s website and search for the latest version of the parameters.Īs you can see, the CHARMM force field has been updated every July for the past several years. In particular, the Alex Mackerell group at the University of Maryland has been actively developing the CHARMM force field parameters. One of the most commonly used parameters for biomolecules is the CHARMM force field, which was originally developed by the Karplus group at Harvard University. Therefore, before starting the MD simulation, the topology information of the target system should also be prepared. However, most MD programs cannot automatically create topology from PDB coordinates due to the complexity of the process. Such information is essential to calculate the sum of each term in the equation. Topology refers to the “atom connectivity” in a molecule, i.e., “which atoms are connected by covalent bonds. In addition, we need information about the “topology” of the target molecular system. Therefore, in order to perform MD simulations, the force field parameters need to be prepared and loaded as input to the MD program. They are also not included in most MD programs. In fact, these physical parameters are not included in any PDB file. We can see many empirical parameters such as force constants ( k b and k a), equilibrium bond length ( r 0), depth of the dihedral angle potential energy ( V n), atomic charge (q), and so on. ![]() This equation is called the “force field”. In the MD simulation, potential energy of the system is calculated by What is contained in the parameter and topology files? ![]()
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