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  2. Perturbation theory (quantum mechanics) - Wikipedia

    en.wikipedia.org/wiki/Perturbation_theory...

    The first-order energy shift is not well defined, since there is no unique way to choose a basis of eigenstates for the unperturbed system. The various eigenstates for a given energy will perturb with different energies, or may well possess no continuous family of perturbations at all.

  3. Isotopic shift - Wikipedia

    en.wikipedia.org/wiki/Isotopic_shift

    Using perturbation theory, the first-order energy shift can be calculated as = >, which requires the knowledge of accurate many-electron wave function. Due to the 1 / M N {\displaystyle 1/M_{N}} term in the expression, the specific mass shift also decrease as 1 / M N 2 {\displaystyle 1/M_{N}^{2}} as mass of nucleus increase, same as normal mass ...

  4. Stark effect - Wikipedia

    en.wikipedia.org/wiki/Stark_effect

    If g = 1 (as is often the case for electronic states of molecules) the first-order energy becomes proportional to the expectation (average) value of the dipole operator , = | | = . Because the electric dipole moment is a vector ( tensor of the first rank), the diagonal elements of the perturbation matrix V int vanish between states that have a ...

  5. Fine structure - Wikipedia

    en.wikipedia.org/wiki/Fine_structure

    The fine structure energy corrections can be obtained by using perturbation theory.To perform this calculation one must add three corrective terms to the Hamiltonian: the leading order relativistic correction to the kinetic energy, the correction due to the spin–orbit coupling, and the Darwin term coming from the quantum fluctuating motion or zitterbewegung of the electron.

  6. Entropy (order and disorder) - Wikipedia

    en.wikipedia.org/wiki/Entropy_(order_and_disorder)

    To highlight the fact that order and disorder are commonly understood to be measured in terms of entropy, below are current science encyclopedia and science dictionary definitions of entropy: A measure of the unavailability of a system's energy to do work; also a measure of disorder; the higher the entropy the greater the disorder. [4]

  7. Bohr model - Wikipedia

    en.wikipedia.org/wiki/Bohr_model

    The two additional assumptions that [1] this X-ray line came from a transition between energy levels with quantum numbers 1 and 2, and [2], that the atomic number Z when used in the formula for atoms heavier than hydrogen, should be diminished by 1, to (Z − 1) 2. Moseley wrote to Bohr, puzzled about his results, but Bohr was not able to help.

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  9. Van 't Hoff equation - Wikipedia

    en.wikipedia.org/wiki/Van_'t_Hoff_equation

    Otherwise, the Van 't Hoff isotherm predicts the direction that the system must shift in order to achieve equilibrium; when Δ r G < 0, the reaction moves in the forward direction, whereas when Δ r G > 0, the reaction moves in the backwards direction. See Chemical equilibrium.