Full Matrix Minimization (cont.)

• The matrix of second derivatives is composed of two parts.

  • The diagonal -- the “mass” of the parameter
  • The off-diagonal -- the correlations between parameters

• Off-diagonal elements arise from

  • the overlap of electron density.
  • geometric restraints.

• The off-diagonal elements are less important for crystallographic data.

  • This results in the sparse matrix method used in PROLSQ, REFMAC and SHELLX.

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