Computational Chemistry CC may take up to 50 % of supercomputer resources, while Computational Solid Mechanics CSM may take less than 10%.
The world of molecules built from atoms and electrons can be seen as a microscopic analog of a macroscopic world built from beams of steel such as a bridge, and so we may expect to find CC as a microscopic analog of CSM, at least in a perspective of classical continuum mechanics.
But this is not what the above numbers show: CC requires vastly more computational work than CSM. Why is that?
Consider a macroscopic object like a bridge composed of $N$ elements, which could be the finite elements in a discretisation of the object in CSM based on Navier's equations of elasticity. The computational work can scale with $N$, that is linearly in $N$ since each element interacts with just a few neighbouring elements. Finite element codes with $N=10^6$ can be run on a lap top.
The situation in CC is vastly different. This is because CC is based on Schrödinger's equation as the basic model of Standard Quantum Mechanics StdQM, which for a molecule with $N$ electrons involves $3N$ spatial dimensions, a full 3d space for each electron. This means that the computational work increases exponentially with $N$ which makes even $N=10$ beyond the power of any thinkable computer, and so only simplified versions of Schrödinger's equation are used in practice. Full solutions named wave functions then appear as pieces of conversation, which have no precise quantitative form.
In Density Functional Theory DFT as the current work horse of CC, Schrödinger's equation is draconically reduced into a 3d equation in a single electrons density. The CSM analog would be to compress a complex bridge construction into one simple beam, where all element individuality is erased.
Electron individuality is thus destroyed in DFT, asking for some form of recovery as electron exchange-correlation, which has shown to be difficult to realise.
RealQM is new methodology starting from a new form of Schrödinger's equation i terms of a collection of a non-overlapping one-electron densities keeping individuality of electrons by spatial occupation. RealQM can be seen as a refined form of DFT with one-electron densities maintaining individuality.
RealQM in principle scales linearly with $N$ just like CSM. Below you can compare a bridge and a molecule and ask yourself why the molecule in CC with StdQM requires vastly more computational work than the bridge in CSM, and so get motivated to take a look at RealQM for which the work is comparable.
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Molecule. |
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Bridge. |