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Lecture 11 REPRESENTATIONS OF SYMMETRY POINT GROUPS 1) Mulliken labels

Lecture 11 REPRESENTATIONS OF SYMMETRY POINT GROUPS 1) Mulliken labels. 2) Character tables. Simplest cases.

wesley-rush
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Lecture 11 REPRESENTATIONS OF SYMMETRY POINT GROUPS 1) Mulliken labels

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  1. Lecture 11REPRESENTATIONS OF SYMMETRY POINT GROUPS1) Mulliken labels

  2. 2) Character tables. Simplest cases • C1 group. Consists of a single operation E; thus its order h=1 and number of classes is 1. There is a single irreducible representation. Its character with any basis function is 1 (E). • Cs group. Consists of two operations, E andsh; thus its order h is 2 and the number of classes is 2. There are two irreducible representations. They can be one-dimensional only, (1)2 + (1)2 = 2. • Ci group. Consists of two operations, E and i. Both its order h and number of classes is 2. Similarly to Cs, the group includes two irreducible one-dimensional representations.

  3. 3) The character table for the C3v point group • Symmetry operations constituting C3v point groupare E, 2 C3 and 3sv (h = 6). • The number of classes is 3. Thus, there are three irreducible representations. One of them is always with all characters equal to 1 (E). Note that 112 + 2(1)2 + 3(1)2 = 6. • The only combination of dimensions of the three representations which squares sum to 6 is 1, 1 and 2 (12 + 12 + 22 = 6). • The second one-dimension representation G2 is orthogonal to the G1: 1(1)(1)+2(1)(1)+3(1)(-1)=0 • The third representation G3 is of order 2. Thus, c3(E) = 2. • Orthogonality of 3rd, 1st and 2nd representations allows us to find c3(C3) = -1 and c3(sv) = 0 and the complete set of characters: 1(1)(2)+2(1)(x)+3(1)(y)=0 (G1┴ G3) 1(1)(2)+2(1)(x)+3(-1)(y)=0 (G2┴ G3) give x = -1 and y = 0

  4. 4) Reducible representations • Any reducible representationGr of a group can always be decomposed into a sum of irreducible ones Gi: • Number of times a particular irreducible representationGi appears in this decomposition, Ni, can be calculated according to the formula: here h is the group order, cr(X) is the character of a symmetry operation X in the reducible representation; ci(X) is the character of X in the irreducible representation i and n(X) is the number of symmetry operations in the class.

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