Flexible molecules

Modelling of internal degrees of freedom, usual techniques:

Bond distances

• Atoms linked by a chemical bond (stretching):

$V_{str}(r_{12})={\frac {1}{2}}K_{str}(r_{12}-b_{0})^{2}$

Bond Angles

Bond sequence: 1-2-3:

Bond Angle: $\theta$

$\cos \theta ={\frac {{\vec {r}}_{21}\cdot {\vec {r}}_{23}}{|{\vec {r}}_{21}||{\vec {r}}_{23}|}}$

Two typical forms are used to model the bending potential:

$V_{bend}(\theta )={\frac {1}{2}}k_{\theta }\left(\theta -\theta _{0}\right)^{2}$

$V_{bend}(\cos \theta )={\frac {1}{2}}k_{c}\left(\cos \theta -c_{0}\right)^{2}$

Dihedral angles. Internal Rotation

Bond sequence: 1-2-3-4

Dihedral angle ($\phi$) definition:

Consider the following vectors:

• ${\vec {a}}\equiv {\frac {{\vec {r}}_{3}-{\vec {r}}_{2}}{|{\vec {r}}_{3}-{\vec {r}}_{2}|}}$; Unit vector in the direction of the 2-3 bond

• ${\vec {b}}\equiv {\frac {{\vec {r}}_{21}-({\vec {r}}_{21}\cdot {\vec {a}}){\vec {a}}}{|{\vec {r}}_{21}-({\vec {r}}_{21}\cdot {\vec {a}}){\vec {a}}|}}$;

Component of ${\vec {r}}_{21}$ that is ortogonal to ${\vec {a}}$ (normalized)

• ${\vec {e}}_{34}\equiv {\frac {{\vec {r}}_{34}-({\vec {r}}_{34}\cdot {\vec {a}}){\vec {a}}}{|{\vec {r}}_{34}-({\vec {r}}_{34}\cdot {\vec {a}}){\vec {a}}|}}$

• ${\vec {c}}={\vec {a}}\times {\vec {b}}$

• $e_{34}=(\cos \phi ){\vec {a}}+(sin\phi ){\vec {c}}$

For molecules with internal rotation degrees of freedom (e.g. n-alkanes), a torsional potential is usually modelled as:

• • $V_{tors}\left(\phi \right)=\sum _{i=0}^{n}a_{i}\left(\cos \phi \right)^{i}$

or

• • $V_{tors}\left(\phi \right)=\sum _{i=0}^{n}b_{i}\cos \left(i\phi \right)$