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The changes made to Version 3.0 to create Version 3.1 are as follows:

- The last digit of the Rydberg constant was changed from 8 to 9.
- The last digit of the Rydberg constant times the speed of light in vacuum (i.e., the Rydberg constant in Hz) was changed from 7 to 8.
- The last digit of the factor for converting an energy expressed in inverse meters to an energy expressed in hartrees was changed from 1 to 0.
- The definition of the weak mixing angle was clarified and its associated footnote d was revised.
- The expression that relates the mass of a particle in the unified atomic mass unit u and its relative atomic mass was added in the "Quantity" column of each entry for the mass of a particle in u.
- Values of quantities given for the positive muon and tau (considered the antiparticles) were changed to values for the negative muon and tau (considered the particles). This had the effect of changing the sign of the g-factor and magnetic moment of the muon as well as the signs of magnetic-moment ratios involving the magnetic moment of the muon. It also led to a change in the definition of the muon magnetic moment anomaly and the muon g-factor.
- The subscript 0 on the symbol for the molar mass constant was changed to a lower case roman u.
- The definitions of the Cu x unit, Mo x unit, and the ångstrom star x-ray units were clarified.
- The symbol for natural units was changed from nu to n.u. and the symbol for atomic units was changed from au to a.u. .
- The form of the tables of energy equivalents was improved.

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