### Get Physical Constant

Source: https://molssi.github.io/QCElemental

Retrieve a physical constant by its name from NIST CODATA. Example: 'hartree energy in ev'.

```python
>>> qcel.constants.get("hartree energy in ev")
27.21138602
```

--------------------------------

### Get Conversion Factor with Pint

Source: https://molssi.github.io/QCElemental

Obtain an arbitrary conversion factor between units using the Pint library. Example: convert 'bohr' to 'miles'.

```python
>>> qcel.constants.conversion_factor("bohr", "miles")
3.2881547429884475e-14
```

--------------------------------

### Get Atomic Mass from Element Symbol

Source: https://molssi.github.io/QCElemental

Retrieve the atomic mass of an element using its symbol (e.g., 'Ne').

```python
>>> qcel.periodictable.to_mass("Ne")
19.9924401762
```

--------------------------------

### Get Atomic Mass from Atomic Number

Source: https://molssi.github.io/QCElemental

Retrieve the atomic mass of an element using its atomic number (e.g., 10 for Neon).

```python
>>> qcel.periodictable.to_mass(10)
19.9924401762
```

--------------------------------

### Parse Molecule from String (PubChem)

Source: https://molssi.github.io/QCElemental

Translate a molecule string into the MolSSI QCSchema format. Supports searching databases like PubChem.

```python
>>> qcel.molparse.from_string("pubchem:benzene")
    Searching PubChem database for benzene (single best match returned)
    Found 1 result(s)
    {"geometry": ...}
```

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