In nuclear magnetic resonance (NMR) spectroscopy, the chemical shift is the resonant frequency of an atomic nucleus relative to a standard in a magnetic field. Often the position and number of chemical shifts are diagnostic of the structure of a molecule. Chemical shifts are also used to describe signals in other forms of spectroscopy such as photoemission spectroscopy. Some atomic nuclei possess a magnetic moment (nuclear spin), which gives rise to different energy levels and resonance frequencies in a magnetic field. The total magnetic field experienced by a nucleus includes local magnetic fields induced by currents of electrons in the molecular orbitals (electrons have a magnetic moment themselves). The electron distribution of the same type of nucleus (e.g. ) usually varies according to the local geometry (binding partners, bond lengths, angles between bonds, and so on), and with it the local magnetic field at each nucleus. This is reflected in the spin energy levels (and resonance frequencies). The variations of nuclear magnetic resonance frequencies of the same kind of nucleus, due to variations in the electron distribution, is called the chemical shift. The size of the chemical shift is given with respect to a reference frequency or reference sample (see also chemical shift referencing), usually a molecule with a barely distorted electron distribution. Operating frequency The operating (or Larmor) frequency of a magnet is calculated from the Larmor equation where is the actual strength of the magnet in units like Teslas or Gauss, and is the gyromagnetic ratio of the nucleus being tested which is in turn calculated from its magnetic moment and spin number with the nuclear magneton and the Planck constant : Thus for example, the proton operating frequency for a 1 T magnet is calculated as: MRI scanners are often referred to by their field strengths (e.g. "a 7 T scanner"), whereas NMR spectrometers are commonly referred to by the corresponding proton Larmor frequency (e.g. "a 300 MHz spectrometer", which has a of 7 T ). While chemical shift is referenced in order that the units are equivalent across different field strengths, the actual frequency separation in Hertz scales with field strength (). As a result, the difference of chemical shift between two signals (ppm) represents a larger number of Hertz on machines that have larger and therefore the signals are less likely to be overlapping in the resulting spectrum. This increased resolution is a significant advantage