Setting up a Gaussian calculation requires careful consideration of multiple parameters to ensure accurate and efficient results. First, you need to select an appropriate computational method and basis set based on your system's size and the properties you wish to study. For small to medium-sized systems, density functional theory (DFT) methods like B3LYP or M06-2X with a 6-31G* basis set offer a good balance between accuracy and computational cost, while for larger systems or higher accuracy requirements, you might consider MP2 or coupled-cluster methods with correlation-consistent basis sets like cc-pVTZ. The molecular geometry should be carefully defined, either through Cartesian coordinates or Z-matrix internal coordinates, including the correct charge and spin multiplicity for your system. For example, a neutral singlet molecule would be specified as "0 1" before listing the atomic coordinates. The calculation type must then be specified in the route section, whether it's a single-point energy calculation, geometry optimization, frequency analysis, or time-dependent DFT for excited states. Additional keywords may be needed for specific applications, such as solvent effects using polarizable continuum models (PCM) or conductor-like screening models (COSMO), dispersion corrections for weak interactions, or stability analysis for problematic wavefunctions. Computational resources should be allocated appropriately using %NProcShared for parallel processing and %Mem for memory allocation. Once the input file is prepared, the calculation is executed through the Gaussian interface, and the output file should be carefully examined for convergence, errors, and the desired molecular properties. Post-processing might involve analyzing optimized geometries, vibrational frequencies, molecular orbitals, or spectroscopic properties, with special attention to potential imaginary frequencies in geometry optimizations that might indicate transition states rather than minima. For complex systems, it may be necessary to perform multiple calculations with increasing levels of theory to ensure result reliability, or to employ techniques like frozen core approximations or resolution of identity to manage computational costs.

Nandish S H

i'll suggest you go through the youtube channel of gaussian 16 and read the manual and may take alook at the joaquin barroso blogs, there are a tons of resource on how to set a gaussian calculation.