Q103 · CSIR-NET Chemistry, December 2013

Paper: CSIR-NET December 2013 · Subject: Physical Chemistry · Chapter: Quantum Chemistry · Topic: Postulates Operators · Marks: 2 · Difficulty: Medium

The quantum mechanical virial theorem for a general potential $\mathrm{V}(\mathrm{x}, \mathrm{y}, \mathrm{z})$ is given by $\left\langle\mathrm{x} \frac{\partial \mathrm{v}}{\partial \mathrm{x}}+\mathrm{y} \frac{\partial \mathrm{v}}{\partial \mathrm{y}}+\mathrm{z} \frac{\partial \mathrm{v}}{\partial \mathrm{z}}\right\rangle$ where T is the kinetic energy operator and <> indicates expectation value. This leads to the following relation between the expectation value of kinetic energy and potential energy for a quantum mechanical harmonic oscillator problem with potential $\mathrm{V}=\frac{1}{2} \mathrm{k}_{\mathrm{x}} \mathrm{x}^{2}+\frac{1}{2} \mathrm{k}_{\mathrm{y}} \mathrm{y}^{2}+\frac{1}{2} \mathrm{k}_{\mathrm{z}} \mathrm{z}^{2}$
(a)$\langle\mathrm{T}\rangle=\langle\mathrm{V}\rangle$
(b)$\langle\mathrm{T}\rangle=-1 / 2\langle\mathrm{V}\rangle$
(c)$\langle\mathrm{T}\rangle=1 / 2\langle\mathrm{V}\rangle$
(d)$\langle\mathrm{T}\rangle=-\langle\mathrm{V}\rangle$
Answer
Answer: A ✓ checked by 4AB · confidence high

The source book printed no answer; this one was worked out and checked — see the explanation.

Explanation
For V ∝ x² + y² + z², Σ x_i ∂V/∂x_i = 2V, so 2⟨T⟩ = 2⟨V⟩ ⇒ ⟨T⟩ = ⟨V⟩.

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