Gaussian Thermochemistry

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Thermochemistry in Gaussian

Joseph W. Ochterski, [email protected]

c2000, Gaussian, Inc.

April 19, 2000

Abstract

The purpose of this paper is to explain how various thermochemical values arecomputed in Gaussian. The paper documents what equations are used to calculate

the quantities, but doesnt explain them in great detail, so a basic understanding

of statistical mechanics concepts, such as partition functions, is assumed. Gaussian

thermochemistry output is explained, and a couple of examples, including calculating

the enthalpy and Gibbs free energy for a reaction, the heat of formation of a molecule

and absolute rates of reaction are worked out.

Contents

1 Introduction 2

2 Sources of components for thermodynamic quantities 2

2.1 Contributions from translation . . . . . . . . . . . . . . . . . . . . . . . . . . 32.2 Contributions from electronic motion . . . . . . . . . . . . . . . . . . . . . . 42.3 Contributions from rotational motion . . . . . . . . . . . . . . . . . . . . . . 42.4 Contributions from vibrational motion . . . . . . . . . . . . . . . . . . . . . 6

3 Thermochemistry output from Gaussian 8

3.1 Output from a frequency calculation . . . . . . . . . . . . . . . . . . . . . . 83.2 Output from compound model chemistries . . . . . . . . . . . . . . . . . . . 11

4 Worked-out Examples 11

4.1 Enthalpies and Free Energies of Reaction . . . . . . . . . . . . . . . . . . . . 124.2 Rates of Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.3 Enthalpies and Free Energies of Formation . . . . . . . . . . . . . . . . . . . 14

5 Summary 17

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1 Introduction

The equations used for computing thermochemical data in Gaussian are equivalent to thosegiven in standard texts on thermodynamics. Much of what is discussed below is coveredin detail in Molecular Thermodynamics by McQuarrie and Simon (1999). Ive cross-

referenced several of the equations in this paper with the same equations in the book, tomake it easier to determine what assumptions were made in deriving each equation. Thesecross-references have the form [McQuarrie, 7-6, Eq. 7.27] which refers to equation 7.27 insection 7-6.

One of the most important approximations to be aware of throughout this analysis isthat all the equations assume non-interacting particles and therefore apply only to an idealgas. This limitation will introduce some error, depending on the extent that any systembeing studied is non-ideal. Further, for the electronic contributions, it is assumed that thefirst and higher excited states are entirely inaccessible. This approximation is generally nottroublesome, but can introduce some error for systems with low lying electronic excited

states.The examples in this paper are typically carried out at the HF/STO-3G level of theory.

The intent is to provide illustrative examples, rather than research grade results.The first section of the paper is this introduction. The next section of the paper, I give the

equations used to calculate the contributions from translational motion, electronic motion,rotational motion and vibrational motion. Then I describe a sample output in the thirdsection, to show how each section relates to the equations. The fourth section consists ofseveral worked out examples, where I calculate the heat of reaction and Gibbs free energy ofreaction for a simple bimolecular reaction, and absoloute reaction rates for another. Finally,an appendix gives a list of the all symbols used, their meanings and values for constants Iveused.

2 Sources of components for thermodynamic quanti-

ties

In each of the next four subsections of this paper, I will give the equations used to calculatethe contributions to entropy, energy, and heat capacity resulting from translational, elec-tronic, rotational and vibrational motion. The starting point in each case is the partitionfunction q(V, T) for the corresponding component of the total partition function. In thissection, Ill give an overview of how entropy, energy, and heat capacity are calculated from

the partition function.The partition function from any component can be used to determine the entropy con-

tribution S from that component, using the relation [McQuarrie, 7-6, Eq. 7.27]:

S= NkB + NkB ln

q(V, T)

N

+ NkBT

ln q

T

V

The form used in Gaussian is a special case. First, molar values are given, so we candivide by n = N/NA, and substitute NAkB = R. We can also move the first term into the

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logarithm (as e), which leaves (with N= 1):

S = R + R ln (q(V, T)) + RT

ln q

T

V

= R ln (q(V, T)e) + RTln q

T

V

= R

ln(qtqeqrqve) + T

ln q

T

V

(1)

The internal thermal energy E can also be obtained from the partition function [Mc-Quarrie, 3-8, Eq. 3.41]:

E= NkBT2

ln q

T

V

, (2)

and ultimately, the energy can be used to obtain the heat capacity [McQuarrie, 3.4,Eq. 3.25]:

CV =ET

N,V

(3)

These three equations will be used to derive the final expressions used to calculate thedifferent components of the thermodynamic quantities printed out by Gaussian.

2.1 Contributions from translation

The equation given in McQuarrie and other texts for the translational partition function is[McQuarrie, 4-1, Eq. 4.6]:

qt =2mkBT

h23/2

V.

The partial derivative of qt with respect to T is:ln qtT

V

=3

2T

which will be used to calculate both the internal energy Et and the third term in Equation 1.The second term in Equation 1 is a little trickier, since we dont know V. However, for

an ideal gas, PV = NRT =

nNA

NAkBT, and V =

kBTP

. Therefore,

qt =2mkBT

h23/2

kBT

P .

which is what is used to calculate qt in Gaussian. Note that we didnt have to make thissubstitution to derive the third term, since the partial derivative has V held constant.

The translational partition function is used to calculate the translational entropy (whichincludes the factor ofe which comes from Stirlings approximation):

St = R

ln(qte) + T

3

2T

= R(ln qt + 1 + 3/2).

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The contribution to the internal thermal energy due to translation is:

Et = NAkBT2

ln q

T

V

= RT2 3

2T

=3

2RT

Finally, the constant volume heat capacity is given by:

Ct =EtT

=3

2R.

2.2 Contributions from electronic motion

The usual electronic partition function is [McQuarrie, 4-2, Eq. 4.8]:

qe = 0e0/kBT + 1e

1/kBT + 2e2/kBT +

where is the degeneracy of the energy level, n is the energy of the n-th level.Gaussian assumes that the first electronic excitation energy is much greater than kBT.

Therefore, the first and higher excited states are assumed to be inaccessible at any tempera-ture. Further, the energy of the ground state is set to zero. These assumptions simplify the

electronic partition function to:qe = 0,

which is simply the electronic spin multiplicity of the molecule.The entropy due to electronic motion is:

Se = R

ln qe + T

ln qeT

V

= R (ln qe + 0) .

Since there are no temperature dependent terms in the partition function, the electronic

heat capacity and the internal thermal energy due to electronic motion are both zero.

2.3 Contributions from rotational motion

The discussion for molecular rotation can be divided into several cases: single atoms, linearpolyatomic molecules, and general non-linear polyatomic molecules. Ill cover each in order.

For a single atom, qr = 1. Since qr does not depend on temperature, the contributionof rotation to the internal thermal energy, its contribution to the heat capacity and itscontribution to the entropy are all identically zero.

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For a linear molecule, the rotational partition function is [McQuarrie, 4-6, Eq. 4.38]:

qr =1

r

T

r

where r = h

2

/8

2

IkB. I is the moment of inertia. The rotational contribution to theentropy is

Sr = R

ln qr + T

ln qrT

V

= R(ln qr + 1).

The contribution of rotation to the internal thermal energy is

Er = RT2

ln qrT

V

= RT2

1T

= RT

and the contribution to the heat capacity is

Cr =

ErT

V

= R.

For the general case for a nonlinear polyatomic molecule, the rotational partition function

is [McQuarrie, 4-8, Eq. 4.56]:

qr =1/2

r

T3/2

(r,xr,yr,z)1/2)

Now we have

ln qT

V

= 32T

, so the entropy for this partition function is

Sr = R

ln qr + T

ln qrT

V

= R(ln qr +3

2).

Finally, the contribution to the internal thermal energy is

Er = RT2

ln qrT

V

= RT2

3

2T

=3

2RT

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only difference between the two references is the additional factor of v,K/2, (which is thezero point vibrational energy) in the equation for the internal energy Ev. In the expressionsfor heat capacity and entropy, this factor disappears since you differentiate with respect totemperature (T).

The total entropy contribution from the vibrational partition function is:

Sv = R

ln(qv) + T

ln q

T

V

= R

ln(qv) + T

K

v,K2T2

+

K

(v,K/T2)ev,K/T

1 ev,K/T

= R

K

v,K2T

+ ln(1 ev,K/T)

+ T

K

v,K2T2

+

K

(v,K/T2)ev,K/T

1 ev,K/T

= R

K

ln(1 ev,K/T) +

K

(v,K/T)ev,K/T

1 ev,K/T

= R

K

v,K/T

ev,K/T 1 ln(1 ev,K/T)

To get from the fourth line to the fifth line in the equation above, you have to multiply byev,K/T

ev,K/T

.The contribution to internal thermal energy resulting from molecular vibration is

Ev = R

K

v,K

1

2+

1

ev,K/T 1

Finally, the contribution to constant volume heat capacity is

Cv = R

K

ev,K/T

v,K/T

ev,K/T 1

2

Low frequency modes (defined below) are included in the computations described above.Some of these modes may be internal rotations, and so may need to be treated separately,depending on the temperatures and barriers involved. In order to make it easier to correct forthese modes, their contributions are printed out separately, so that they may be subtractedout. A low frequency mode in Gaussian is defined as one for which more than five percent ofan assembly of molecules are likely to exist in excited vibrational states at room temperature.In other units, this corresponds to about 625 cm1, 1.91013 Hz, or a vibrational temperatureof 900 K.

It is possible to use Gaussian to automatically perform some of this analysis for you, viathe Freq=HindRot keyword. This part of the code is still undergoing some improvements, soI will not go into it in detail. See P. Y. Ayala and H. B. Schlegel, J. Chem. Phys. 108 2314(1998) and references therein for more detail about the hindered rotor analysis in Gaussianand methods for correcting the partition functions due to these effects.

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Compound E0 E0 + GcorrHF 98.572847 98.579127HD 98.572847 98.582608Cl 454.542193 454.557870

HCl 455.136012 455.146251

DCl 455.136012 455.149092F 97.986505 98.001318

FHCl 553.090218 553.109488FDCl 553.090218 553.110424

Table 2: Total HF/STO-3G electronic energies and sum of electronic and thermal free ener-gies for atoms molecules and transition state complex of the reaction FH + Cl F + HCland the deuterium substituted analog.

A Molecular Approach by D. A. McQuarrie and J. D. Simon. The key equation (number28.72, in that text) for calculating reaction rates is

k(T) =kBT

hce

G/RT

Ill use c = 1 for the concentration. For simple reactions, the rest is simply plugging inthe numbers. First, of course, we need to get the numbers. Ive run HF/STO-3G frequencycalculation for reaction FH + Cl F + HCl and also for the reaction with deuteriumsubstituted for hydrogen. The results are summarized in Table 2. I put the total electronicenergies of each compound into the table as well to illustrate the point that the final geometryand electronic energy are independent of the masses of the atoms. Indeed, the cartesianforce constants themselves are independent of the masses. Only the vibrational analysis,and quantities derived from it, are mass dependent.

The first step in calculating the rates of these reactions is to compute the free energy ofactivation, G ((H) is for the hydrogen reaction, (D) is for deuterium reaction):

G(H) = 553.109488 (98.579127 + 454.557870)

= 0.027509 Hartrees

= 0.027509 627.5095 = 17.26 kcal/mol

G(D) = 553.110424 (98.582608 + 454.557870)

= 0.030054 Hartrees

= 0.030054 627.5095 = 18.86 kcal/mol

Then we can calculate the reaction rates. The values for the constants are listed in theappendix. Ive taken c = 1.

k(298, H) =kBT

hce

G/RT

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=1.380662 1023(298.15)

6.626176 1034(1)exp

17.26

1.987 298.15

= 6.2125 1012e0.02913

= 6.034 1012s1

k(298, D) = 6.2125 1012 exp

18.86

1.987 298.15

= 6.2125 1012e0.031835

= 6.017 1012s1

So we see that the deuterium reaction is indeed slower, as we would expect. Again, thesecalculations were carried out at the HF/STO-3G level, for illustration purposes, not for re-search grade results. More complex reactions will need more sophisticated analyses, perhaps

including careful determination of the effects of low frequency modes on the transition state,and tunneling effects.

4.3 Enthalpies and Free Energies of Formation

Calculating enthalpies of formation is a straight-forward, albeit somewhat tedious task, whichcan be split into a couple of steps. The first step is to calculate the enthalpies of formation(fH

(0K)) of the species involved in the reaction. The second step is to calculate theenthalpies of formation of the species at 298K.

Calculating the Gibbs free energy of reaction is similar, except we have to add in theentropy term:

fG(298) = fH

(298K) T(S(M, 298K)

S(X, 298K))

To calculate these quantities, we need a few component pieces first. In the descriptionsbelow, I will use M to stand for the molecule, and X to represent each element which makesup M, and x will be the number of atoms ofX in M.

Atomization energy of the molecule,D0(M):

These are readily calculated from the total energies of the molecule (E0(M)), the zero-point energy of the molecule (EZPE(M)) and the constituent atoms:

D0(M) =atoms

xE0(X) E0(M) + EZPE(M)

Heats of formation of the atoms at 0K, fH(X, 0K):

I have tabulated recommended values for the heats of formation of the first and secondrow atomic elements at 0K in Table 3. There are (at least) two schools of thought withrespect to the which atomic heat of formation data is most appropriate. Some authorsprefer to use purely experimental data for the heats of formation (Curtiss, et. al., J.

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r = symmetry number for rotation

h = Plancks constant = 6.626176 1034 J s

m = mass of the molecule

n = number of particles (always 1)

qe = electronic partition function

qr = rotational partition function

qt = translational partition function

qv = vibrational partition function

EZPE = zero-point energy of the molecule

E0 = the total electronic energy, the MP2 energy for example

H, S, G = standard enthalpy, entropy and Gibbs free energy each compound is in itsstandard state at the given temperature

G = Gibbs free energy of activation

c = concentration (taken to be 1)

k(T) = reaction rate at temperature T

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