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The memory available to HyperChem is the sum of random access memory (RAM) and swap space, or virtual memory. The latter is a section of a hard disk set aside for use by programs during their execution. Virtual memory is much slower than RAM, as disk access is required. However, the size of calculation that can be done depends only on the total memory available, RAM + swap space. In Windows, the size of your swap space is set in the Control Panel application, that is part of the "Main" application group. Choose the "386-Enhanced" item, and click on the "Virtual Memory" button to set this size.
The total memory that needs to be available in order to carry out a calculation using HyperChem is as follows:
1. Memory required by Windows, DOS and items specified in your
config.sys and autoexec.bat files, such as Smartdrive.
2. Memory required by the HyperChem executable (CHEM.EXE)
3. Memory required by the computational "backend"
executable (NDO.EXE, EHT.EXE, MMPLUS.EXE or NEWTON.EXE)
4. Memory required to carry out the particular calculation.
Items 1, 2, and 3 are independent of the molecular system under
study. Item 4 depends on the particular calculation you are
carrying out. You should note that any other applications running
will also consume memory: when doing large calculations it may be
necessary to ensure that you are not using other Windows
applications.
If you click on the "About" item under the Help menu of Program Manager, you will see the amount of memory currently available to you. In general, this does NOT add up to the total of your RAM + swap space, but is substantially less. On a machine with 8 Mb RAM and 16 Mb of swap space, (which "should" have 24 Mb free) the value may be about 18 Mb. The difference is due to drivers and other items loaded in your config.sys and autoexec.bat files, as well as the basic DOS and Windows operating system requirements.
While running any calculation in HyperChem, memory is needed to load both the "front end" program (CHEM.EXE) and the computational "back end" program into memory.
Fixed Memory Requirements for HyperChem Programs
===================================================
Program Calculation Type Memory Required
/ KB
---------------------------------------------------
HyperChem All 640
(CHEM.EXE)
EHT.EXE Extended Huckel 670
NDO.EXE Other semi- 2,500
empirical
MMPLUS.EXE MM+ 350
NEWTON.EXE Other molecular 200
mechanics
===================================================
To find the memory available for a particular calculation, use the following procedure.
1. With no applications running in Windows, look at the About
box in Program Manager to find the available memory.
2. Subtract 640 KB to account for loading HyperChem.
3. Subtract the amount in the table above for loading the program
appropriate for the method you are using.
The following figures are for AM1 or PM3 calculations. These methods are the most demanding of the semi- empirical methods: calculations on a similar size molecule using CNDO requires significantly less memory.
The major contribution to memory usage (not the only one) for a 'standard' RHF calculation is 104 * (Number of orbitals)^2 bytes. If you are doing other kinds of calculation, additional memory is required: UHF: + 40 * (Number of orbitals)^2 bytes Convergence accelerator: + 160 * (Number of orbitals)^2 bytes
HyperChem computes the gradient for all kinds of calculation, so even for a single point calculation to complete, you will need space for a gradient evaluation. The gradient requirements are in addition to the SCF requirements.
In the following expression, NHA = "non-hydrogen atoms", HA = "Hydrogen atoms".
Memory required = 2400 * NHA * (NHA - 1) + 480 * NHA * HA + 24 * HA * (HA-1) bytes.
For most systems, this is substantially less than the memory required for the SCF calculation. For systems with no hydrogens in, however (e.g., C60) the gradient requirements are comparable to that of the SCF.
The "zero" in the following examples is the value given in the Program Manager Help with HyperChem running and NDO loaded. All calculations are AM1 RHF single points, with SCF convergence acceleration unchecked.
NDO Memory Requirements
==========================================
Molecular formula # of Extra memory
orbitals used
------------------------------------------
CH4 (methane) 8 0 Mb
C10H10 50 0.8 Mb
(naphthalene)
C16H10 (pyrene) 74 1.3 Mb
C24H12 108 2.5 Mb
(coronene)
C30H18 138 4 Mb
C40H22 182 8 Mb
C48H24 216 9.5 Mb
==========================================
Significant disk activity, and consequent slowing of the calculation, starts at around the amount of RAM in the computer.
Extended Huckel calculations require significantly less memory than NDO calculations, because they do not include any three-electron integrals. The main data storage involves the orbital coefficients and various square matrices, such as the Hamiltonian and overlap matrices, each of which have Nē elements (N being the number of orbitals in the problem). The memory usage is proportional to the square of the number of orbitals involved in the calculation. Typical cases are shown in the figure.
Extended Huckel Memory Requirements
===============================
Number of Orbitals Memory / Mb
-------------------------------
0 0
90 0.4
180 1.5
270 3.4
360 6
===============================
The memory requirements for the MMPlus program are proportional to the square of the number of atoms in the system. MMPlus keeps a list of all interactions in the system, and as each atom interacts with all others, this dictates a quadratic dependency. The graph below indicates requirements in addition to the static requirements described above.
MM+ Memory Requirements
======================
Number of Memory
atoms Used (Mbytes)
----------------------
10 0.58
60 0.58
258 0.8
652 2
904 3.2
1304 5.2
1956 9.7
======================
The Newton program is more efficient than MMPlus in its memory requirements (as well as in speed) and maintains only information about each atom, not about each interaction. Consequently, the memory requirements for Newton are proportional to the number of atoms. The following graph indicates the requirements in addition to the static requirements outlined above.
Newton Memory Requirements
======================
Number of Memory
atoms Used (Mbytes)
----------------------
472 0.74
944 1.1
1888 1.8
3776 3
======================
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