Ab Initio
Definition and meaning of Ab Initio in chemistry.
Ab initio, Latin for from the beginning, describes computational methods that calculate molecular properties directly from quantum mechanics. These methods use no experimental data as input, relying only on physical constants and atomic positions and charges. This makes them a first principles approach to predicting chemistry.
In more detail
Ab initio methods work by solving approximate versions of the Schrodinger equation. This is the fundamental equation that describes how electrons behave in atoms and molecules. An exact solution is only possible for the simplest systems, like a single hydrogen atom.
Chemists use carefully designed approximations for anything larger. The best-known starting point is the Hartree-Fock method. It treats each electron as moving in the average electric field created by all the other electrons and nuclei in the molecule.
Hartree-Fock provides a reasonable but imperfect picture. It does not fully capture how electrons instantaneously avoid each other, an effect called electron correlation. More advanced ab initio methods build on Hartree-Fock to add back this missing correlation.
This improves accuracy but costs much greater computing time. Ab initio calculations require enormous computational resources for larger molecules. Because of this, chemists often weigh accuracy against practicality when choosing a method.
Semi-empirical methods offer a faster alternative. They plug in parameters fitted from real experimental data, sacrificing some rigor for speed. This makes them useful for larger molecules where full ab initio calculations would take too long.
A common misconception is that ab initio results are always more trustworthy than experimental measurements. In practice, ab initio methods are validated against experiment. They can still contain systematic errors from the approximations they rely on.
Key facts
| Meaning | From the beginning (Latin) |
|---|---|
| Based on | Quantum mechanics, solving the Schrodinger equation |
| Example method | Hartree-Fock |
| Missing effect in Hartree-Fock | Electron correlation |
| Faster alternative | Semi-empirical methods, using fitted experimental parameters |
| Field | Computational chemistry |
A chemist might use an ab initio calculation to predict the shape and energy of a small molecule. This molecule may have never been made in a laboratory. The prediction can guide experimentalists toward promising synthesis targets before any lab work begins.
Frequently asked questions
What does ab initio mean in chemistry?
It refers to calculating molecular properties purely from quantum-mechanical first principles, without using experimental measurements as input, aside from fundamental physical constants.
How is ab initio different from semi-empirical methods?
Ab initio methods use no fitted experimental parameters and are generally more accurate but slower. Semi-empirical methods include parameters fitted to experimental data, running faster with somewhat less accuracy.
Why can't chemists solve the Schrodinger equation exactly for most molecules?
The equation becomes mathematically unsolvable once more than one electron is involved, due to the complex way electrons interact with each other. Chemists instead use careful approximations that get closer to the exact answer.