When chemists calculate the behavior of molecules on a computer, they use approximate methods because exact calculations are too complex. For light atoms, there are well-established benchmarks, but for heavy atoms, where relativistic effects matter, such benchmarks did not exist. It's like checking a ruler against another ruler rather than against a standard meter.

The new approach uses an expansion that allows solving equations for large systems exactly. With its help, scientists obtained benchmark data for molecules with heavy atoms. Then they compared two popular approximate methods against these data and found out how much they err.

Such checks help understand where approximate methods are reliable and where they fail. This is important for developing new materials and drugs, where prediction accuracy is critical.