I'm planning to do some molecular dynamic simulation with Gaussian, (trying to simulate the alignment of an ensemble of water molecules in the presence of strong field), can anyone please explain if that is possible with Gaussian?
Yes, with some limitations. Gaussian can carry out classical trajectory calculations by means of the Atom Centered Density Matrix Propagation MD model (ADMP, https://gaussian.com/admp) and the – more computationally expensive – Born-Oppenheimer MD model (BOMD).
Different software suites, such as Gromacs and AMBER, are more commonly used for MD simulations.