Computational Chemistry vs Computational Resources

Ab initio calculations, which are quantum mechanical calculations performed without any experimentally determined input, require significant computational power. Ab initio calculations are called a Hartree-Fock (HF) calculation, where the primary approximation is the central field approximation in which the electron-electron repulsion is calculated by integrating the repulsion term. Thus, only the average effect of the repulsion is obtained, not the exact explicit repulsion interaction. The HF method scales as N⁴, where N is the number of basis functions. The scaling of ab initio calculations with computational power can be broken down into three main areas:

1. System size: The larger the system being studied, the more computational power is required. This is because more atoms or molecules lead to a more complex system that requires more computational resources to model. For example, a small molecule like water may only require a few basis functions to model accurately, whereas a larger molecule like DNA could require millions of basis functions.
2. Methodology: The level of theory used to perform the computational calculations affects the computational resources required. Simple methods may be computationally inexpensive, whereas more advanced methods like density functional theory (DFT) or Coupled-Cluster theory (CC) require significantly more computational resources. In DFT methods, the energy of a molecule can be determined from the electron density instead of a wave function. DFT tends to be classified either as an ab initio method or in a class by itself. The advantage of using electron density is that the integrals for Coulomb repulsion need to be done only over the electron density, which is a three-dimensional function, thus scaling as N³.
3. Accuracy: Lastly, the accuracy desired in the results of the calculation also affects computational requirements. Achieving high accuracy requires the use of more computationally demanding methods and larger system sizes.

Performing computational calculations, such as Ab initio or DFT, requires significant computational resources. The required resources depend on various factors, including the complexity of the system being modeled, the level of theory selected, and the desired accuracy and precision. These calculations require access to high-performance computing (HPC) resources, such as a supercomputer or a high-performance cluster. The exact hardware requirements depend on the size and complexity of the system being studied. Large computational resources, such as multicore CPUs or graphics processing units (GPUs), are required to perform calculations on large molecular systems or materials.

Computational chemistry calculations require specialized software packages, such as Gaussian, VASP, Quantum ESPRESSO, or NWChem, specifically designed to perform calculations. These software packages require skilled personnel to install and maintain them and also require licenses. Some of the major computational resources needed for calculations are:

1. Processor: A high-performance computer or cluster with multiple processors is typically necessary to perform computational calculations quickly and efficiently. The processor should also have a high clock speed, as calculations are often computationally intensive and can take a long time to complete.
2. Memory (RAM): Computational calculations require a lot of memory (RAM) to store the large matrices and arrays involved in solving the Schrödinger equation. The amount of RAM required depends on the size and complexity of the system, as well as the desired accuracy of the results. Computational calculations require a significant amount of memory (RAM) to store the electron density, orbitals, and other variables used in the calculations. Thus, the amount of memory required depends on the size and complexity of the system being studied.
3. Storage: Computational calculations generate large amounts of data, including input and output files, as well as intermediate results. Adequate storage space is necessary to store and manage these files. For example, Gaussian log files of calculations, such as MP2 and coupled clusters, require a large amount of disk space to store these files. Effective storage systems are needed to store these large amounts of data.
4. Software: A suite of computational chemistry software packages, such as Gaussian or NWChem, is needed to perform computational chemistry calculations. These software packages vary in terms of their computational efficiency, user interface, and available functionality.
5. Access to supercomputers: For large-scale calculations, access to supercomputers may be necessary, as the computational resources required can exceed the capacity of most desktop or laptop computers. Supercomputers are specialized high-performance computing platforms that provide vast amounts of computational power and memory to researchers.

Overall, Computational chemistry calculations require a significant amount of energy to power the computing hardware and cooling systems.
For example, ab initio calculations scale exponentially with computational power. As system size, methodology, and accuracy requirements increase, so does the computational cost. With recent advancements in computing hardware and algorithms, however, the ability to perform increasingly complex ab initio calculations has improved significantly.
Additionally, advances in computer hardware and software optimization also contribute to the scalability of ab initio calculations. Increasing computational power enables scientists to explore complex molecules and materials, which were previously considered infeasible or unattainable. This advancement in computational power has revolutionized many fields of science, including material science, chemistry, and biology.