# Using VMD plug-ins for trajectory file I/O#

GROMACS tools are able to use the plug-ins found in an existing installation of VMD in order to read and write trajectory files in formats that are not native to GROMACS. You will be able to supply an AMBER DCD-format trajectory filename directly to GROMACS tools, for example.

This requires a VMD installation not older than version 1.8, that your system provides the dlopen function so that programs can determine at run time what plug-ins exist, and that you build shared libraries when building GROMACS. CMake will find the vmd executable in your path, and from it, or the environment variable VMDDIR at configuration or run time, locate the plug-ins. Alternatively, the VMD_PLUGIN_PATH can be used at run time to specify a path where these plug-ins can be found. Note that these plug-ins are in a binary format, and that format must match the architecture of the machine attempting to use them.

# Interactive Molecular Dynamics#

GROMACS supports the interactive molecular dynamics (IMD) protocol as implemented by VMD to control a running simulation in NAMD. IMD allows to monitor a running GROMACS simulation from a VMD client. In addition, the user can interact with the simulation by pulling on atoms, residues or fragments with a mouse or a force-feedback device. Additional information about the GROMACS implementation and an exemplary GROMACS IMD system can be found on this homepage.

## Simulation input preparation#

The GROMACS implementation allows transmission and interaction with a part of the running simulation only, e.g. in cases where no water molecules should be transmitted or pulled. The group is specified via the mdp option IMD-group. When IMD-group is empty, the IMD protocol is disabled and cannot be enabled via the switches in mdrun. To interact with the entire system, IMD-group can be set to System. When using grompp, a gro file to be used as VMD input is written out (-imd switch of grompp).

## Starting the simulation#

Communication between VMD and GROMACS is achieved via TCP sockets and thus enables controlling an mdrun running locally or on a remote cluster. The port for the connection can be specified with the -imdport switch of mdrun, 8888 is the default. If a port number of 0 or smaller is provided, GROMACS automatically assigns a free port to use with IMD.

Every $$N$$ steps, the mdrun client receives the applied forces from VMD and sends the new positions to the client. VMD permits increasing or decreasing the communication frequency interactively. By default, the simulation starts and runs even if no IMD client is connected. This behavior is changed by the -imdwait switch of mdrun. After startup and whenever the client has disconnected, the integration stops until reconnection of the client. When the -imdterm switch is used, the simulation can be terminated by pressing the stop button in VMD. This is disabled by default. Finally, to allow interacting with the simulation (i.e. pulling from VMD) the -imdpull switch has to be used. Therefore, a simulation can only be monitored but not influenced from the VMD client when none of -imdwait, -imdterm or -imdpull are set. However, since the IMD protocol requires no authentication, it is not advisable to run simulations on a host directly reachable from an insecure environment. Secure shell forwarding of TCP can be used to connect to running simulations not directly reachable from the interacting host. Note that the IMD command line switches of mdrun are hidden by default and show up in the help text only with gmx mdrun -h -hidden.

## Connecting from VMD#

In VMD, first the structure corresponding to the IMD group has to be loaded (File $$\rightarrow$$ New Molecule). Then the IMD connection window has to be used (Extensions $$\rightarrow$$ Simulation $$\rightarrow$$ IMD Connect (NAMD)). In the IMD connection window, hostname and port have to be specified and followed by pressing Connect. Detach Sim allows disconnecting without terminating the simulation, while Stop Sim ends the simulation on the next neighbor searching step (if allowed by -imdterm).

The timestep transfer rate allows adjusting the communication frequency between simulation and IMD client. Setting the keep rate loads every $$N^\mathrm{th}$$ frame into VMD instead of discarding them when a new one is received. The displayed energies are in SI units in contrast to energies displayed from NAMD simulations.s