# Density Functional Theory Study on the Adsorption of Co(II) in Aqueous Solution by Graphene Oxide
## 2 Theoretical Calculation Method
### Gaussian Calculation Software  
In order to solve the equation more accurately and quickly, various types of density functional computing programs have appeared. At present, the widely used chemical calculation software that can be used for DFT calculation include VASP [6], ORCA [7] and Gaussian [8]. These software can efficiently and quickly perform simulation calculations under microscopic conditions, so that density functional theory is increasingly applied to the research of material design and synthesis, macromolecules and biological systems, etc. Among them, Gaussian is the most widely used quantum chemistry software for the theoretical calculation and ab initio calculation of quantum chemistry, which has a powerful quantum chemistry calculation function. It can be used to calculate the structure and energy of the system, molecular orbital, polarization and hyperpolarization, vibration frequency, infrared and Raman spectra, magnetic properties, etc. At the same time, it can also predict the chemical reaction path and calculate the transition-state energy and structure and other functions. Gaussian 16W has a strong ability to calculate aperiodic structures compared with other calculation software. Graphene oxide is composed of graphene with a periodic structure and rich oxygen-containing functional groups on the surface. In addition, considering the possible grafting structure of derivatives, the periodicity of material molecules will be destroyed. Therefore, the adsorption mechanism of graphene and its derivatives was investigated by using Gaussian software in this paper.  
When using Gaussian software, you can use the GaussView GUI to draw the Gaussian molecular structure diagram and create the input file.gif that contains the molecular coordinates, method and group information. Then, the input file can be imported to the Gaussian calculation and the system information can be analyzed based on the obtained output file. Gaussian software covers many types of calculation methods and basis sets to make it suitable for the calculation of different types of chemical systems. In the process of simulation calculation using Gaussian software, how to choose the appropriate method and basis group becomes the key to the accuracy of calculation results. In the process of Gaussian development, a variety of calculation methods have emerged, such as the semi-empirical method, self-consistent field theory, ordinary functional, double-hybrid functional and single-double iterative coupled cluster theory, etc. Compared with other types of methods, the ordinary functional has certain calculation accuracy, a moderate calculation amount, takes a short amount of time and is the most widely used [9]. The B3 LYP method in the ordinary functional, which combines the Beeke-type three-parameter method [10] and the Lee-Yang-Part functional [11], is the most-used functional in quantum chemistry calculations and can be used on most systems. However, this method cannot describe dispersion attraction in van der Waals forces, so dispersion correction is required during parameter setting.  
The base group is the wave function of the orbit of the system. The larger the base group, the smaller the approximation between the calculation result and the actual situation, and the higher the calculation accuracy, but the calculation amount and calculation time will also be greatly increased. In order to balance the workload and accuracy, Gaussian basis sets commonly used include the splitting valence group, polarization group, dispersion group, minimum group and high angular momentum group. Among them, the polarization group and dispersion group are usually used together with the splitting valence group. For example, 6-31g(d),6-311+g(d,p),6-311+g(2d,2p), etc. When transition-metal elements are involved in the system, specific base groups such as landl2 dz, lanlztz, lanlztz(f) or DEF2-TZVP can also be used for calculations.